diff --git a/metrics-exporter-prometheus/Cargo.toml b/metrics-exporter-prometheus/Cargo.toml
index 0466d8f..773aa82 100644
--- a/metrics-exporter-prometheus/Cargo.toml
+++ b/metrics-exporter-prometheus/Cargo.toml
@@ -25,6 +25,7 @@ metrics-util = { version = "0.4.0-alpha.1", path = "../metrics-util" }
 hdrhistogram = "7.1"
 parking_lot = "0.11"
 thiserror = "1.0"
+quanta = "0.6"
 
 # Optional
 hyper = { version = "0.13", default-features = false, features = ["tcp"], optional = true }
diff --git a/metrics-exporter-prometheus/examples/prometheus_server.rs b/metrics-exporter-prometheus/examples/prometheus_server.rs
index 3791d27..27f2841 100644
--- a/metrics-exporter-prometheus/examples/prometheus_server.rs
+++ b/metrics-exporter-prometheus/examples/prometheus_server.rs
@@ -11,6 +11,7 @@ fn main() {
 
     let builder = PrometheusBuilder::new();
     builder
+        .idle_timeout(Some(Duration::from_secs(10)))
         .install()
         .expect("failed to install Prometheus recorder");
 
@@ -32,6 +33,8 @@ fn main() {
     let mut clock = Clock::new();
     let mut last = None;
 
+    increment!("idle_metric");
+
     // Loop over and over, pretending to do some work.
     loop {
         increment!("tcp_server_loops", "system" => "foo");
diff --git a/metrics-exporter-prometheus/src/lib.rs b/metrics-exporter-prometheus/src/lib.rs
index bc0c8b0..3c4bb5d 100644
--- a/metrics-exporter-prometheus/src/lib.rs
+++ b/metrics-exporter-prometheus/src/lib.rs
@@ -12,13 +12,16 @@ use metrics::{Key, Recorder, SetRecorderError, Unit};
 use metrics_util::{
     parse_quantiles, CompositeKey, Handle, Histogram, MetricKind, Quantile, Registry,
 };
-use parking_lot::RwLock;
+use parking_lot::{Mutex, RwLock};
+use quanta::{Clock, Instant};
 use std::io;
 use std::iter::FromIterator;
 use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+use std::ops::DerefMut;
 use std::sync::Arc;
 #[cfg(feature = "tokio-exporter")]
 use std::thread;
+use std::time::Duration;
 use std::{collections::HashMap, time::SystemTime};
 use thiserror::Error as ThisError;
 #[cfg(feature = "tokio-exporter")]
@@ -66,8 +69,10 @@ struct Snapshot {
     pub distributions: HashMap<String, HashMap<Vec<String>, Distribution>>,
 }
 
-struct Inner {
+pub(crate) struct Inner {
     registry: PrometheusRegistry,
+    recency: Mutex<(Clock, HashMap<CompositeKey, (usize, Instant)>)>,
+    idle_timeout: Option<Duration>,
     distributions: RwLock<HashMap<String, HashMap<Vec<String>, Distribution>>>,
     quantiles: Vec<Quantile>,
     buckets: Vec<u64>,
@@ -80,9 +85,44 @@ impl Inner {
         &self.registry
     }
 
+    fn should_store(
+        &self,
+        key: &CompositeKey,
+        current_gen: usize,
+        clock: &mut Clock,
+        recency: &mut HashMap<CompositeKey, (usize, Instant)>,
+    ) -> bool {
+        if let Some(idle_timeout) = self.idle_timeout {
+            let now = clock.now();
+            if let Some((last_gen, last_update)) = recency.get_mut(&key) {
+                // If the value is the same as the latest value we have internally, and
+                // we're over the idle timeout period, then remove it and continue.
+                if *last_gen == current_gen {
+                    if (now - *last_update) > idle_timeout {
+                        // If the delete returns false, that means that our generation counter is
+                        // out-of-date, and that the metric has been updated since, so we don't
+                        // actually want to delete it yet.
+                        if self.registry.delete(&key, current_gen) {
+                            return false;
+                        }
+                    }
+                } else {
+                    // Value has changed, so mark it such.
+                    *last_update = now;
+                }
+            } else {
+                recency.insert(key.clone(), (current_gen, now));
+            }
+        }
+
+        true
+    }
+
     fn get_recent_metrics(&self) -> Snapshot {
         let metrics = self.registry.get_handles();
 
+        let mut rg = self.recency.lock();
+        let (clock, recency) = rg.deref_mut();
         let mut counters = HashMap::new();
         let mut gauges = HashMap::new();
 
@@ -93,64 +133,76 @@ impl Inner {
             .unwrap_or_else(|| vec![]);
         sorted_overrides.sort_by(|(a, _), (b, _)| b.len().cmp(&a.len()));
 
-        for (key, handle) in metrics.into_iter() {
-            let (kind, key) = key.into_parts();
-            let (name, labels) = key_to_parts(key);
-
-            match kind {
+        for (key, (gen, handle)) in metrics.into_iter() {
+            match key.kind() {
                 MetricKind::Counter => {
-                    let entry = counters
-                        .entry(name)
-                        .or_insert_with(|| HashMap::new())
-                        .entry(labels)
-                        .or_insert(0);
-
-                    *entry = handle.read_counter();
+                    let value = handle.read_counter();
+                    if self.should_store(&key, gen, clock, recency) {
+                        let (_, key) = key.into_parts();
+                        let (name, labels) = key_to_parts(key);
+                        let entry = counters
+                            .entry(name)
+                            .or_insert_with(|| HashMap::new())
+                            .entry(labels)
+                            .or_insert(0);
+                        *entry = value;
+                    }
                 }
                 MetricKind::Gauge => {
-                    let entry = gauges
-                        .entry(name)
-                        .or_insert_with(|| HashMap::new())
-                        .entry(labels)
-                        .or_insert(0.0);
-
-                    *entry = handle.read_gauge();
+                    let value = handle.read_gauge();
+                    if self.should_store(&key, gen, clock, recency) {
+                        let (_, key) = key.into_parts();
+                        let (name, labels) = key_to_parts(key);
+                        let entry = gauges
+                            .entry(name)
+                            .or_insert_with(|| HashMap::new())
+                            .entry(labels)
+                            .or_insert(0.0);
+                        *entry = value;
+                    }
                 }
                 MetricKind::Histogram => {
-                    let buckets = sorted_overrides
-                        .iter()
-                        .find(|(k, _)| name.ends_with(*k))
-                        .map(|(_, buckets)| *buckets)
-                        .unwrap_or(&self.buckets);
+                    if self.should_store(&key, gen, clock, recency) {
+                        let (_, key) = key.into_parts();
+                        let (name, labels) = key_to_parts(key);
 
-                    let mut wg = self.distributions.write();
-                    let entry = wg
-                        .entry(name.clone())
-                        .or_insert_with(|| HashMap::new())
-                        .entry(labels)
-                        .or_insert_with(|| match buckets.is_empty() {
-                            false => {
-                                let histogram = Histogram::new(buckets)
-                                    .expect("failed to create histogram with buckets defined");
-                                Distribution::Histogram(histogram)
-                            }
-                            true => {
-                                let summary =
-                                    HdrHistogram::new(3).expect("failed to create histogram");
-                                Distribution::Summary(summary, 0)
-                            }
-                        });
+                        let buckets = sorted_overrides
+                            .iter()
+                            .find(|(k, _)| name.ends_with(*k))
+                            .map(|(_, buckets)| *buckets)
+                            .unwrap_or(&self.buckets);
 
-                    match entry {
-                        Distribution::Histogram(histogram) => handle
-                            .read_histogram_with_clear(|samples| histogram.record_many(samples)),
-                        Distribution::Summary(summary, sum) => {
-                            handle.read_histogram_with_clear(|samples| {
-                                for sample in samples {
-                                    let _ = summary.record(*sample);
-                                    *sum += *sample;
+                        let mut wg = self.distributions.write();
+                        let entry = wg
+                            .entry(name.clone())
+                            .or_insert_with(|| HashMap::new())
+                            .entry(labels)
+                            .or_insert_with(|| match buckets.is_empty() {
+                                false => {
+                                    let histogram = Histogram::new(buckets)
+                                        .expect("failed to create histogram with buckets defined");
+                                    Distribution::Histogram(histogram)
                                 }
-                            })
+                                true => {
+                                    let summary =
+                                        HdrHistogram::new(3).expect("failed to create histogram");
+                                    Distribution::Summary(summary, 0)
+                                }
+                            });
+
+                        match entry {
+                            Distribution::Histogram(histogram) => {
+                                handle.read_histogram_with_clear(|samples| {
+                                    histogram.record_many(samples)
+                                })
+                            }
+                            Distribution::Summary(summary, sum) => handle
+                                .read_histogram_with_clear(|samples| {
+                                    for sample in samples {
+                                        let _ = summary.record(*sample);
+                                        *sum += *sample;
+                                    }
+                                }),
                         }
                     }
                 }
@@ -371,6 +423,7 @@ pub struct PrometheusBuilder {
     listen_address: SocketAddr,
     quantiles: Vec<Quantile>,
     buckets: Vec<u64>,
+    idle_timeout: Option<Duration>,
     buckets_by_name: Option<HashMap<String, Vec<u64>>>,
 }
 
@@ -383,6 +436,7 @@ impl PrometheusBuilder {
             listen_address: SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 9000),
             quantiles,
             buckets: vec![],
+            idle_timeout: None,
             buckets_by_name: None,
         }
     }
@@ -422,6 +476,17 @@ impl PrometheusBuilder {
         self
     }
 
+    /// Sets the idle timeout for metrics.
+    ///
+    /// If a metric hasn't been updated within this timeout, it will be removed from the registry
+    /// and in turn removed from the normal scrape output until the metric is emitted again.  This
+    /// behavior is driven by requests to generate rendered output, and so metrics will not be
+    /// removed unless a request has been made recently enough to prune the idle metrics.
+    pub fn idle_timeout(mut self, timeout: Option<Duration>) -> Self {
+        self.idle_timeout = timeout;
+        self
+    }
+
     /// Sets the buckets for a specific metric, overidding the default.
     ///
     /// The match is suffix-based, and the longest match found will be used.
@@ -470,8 +535,14 @@ impl PrometheusBuilder {
     /// Builds the recorder and returns it.
     /// This function is only enabled when default features are not set.
     pub fn build(self) -> Result<PrometheusRecorder, Error> {
+        self.build_with_clock(Clock::new())
+    }
+
+    pub(crate) fn build_with_clock(self, clock: Clock) -> Result<PrometheusRecorder, Error> {
         let inner = Arc::new(Inner {
             registry: Registry::new(),
+            recency: Mutex::new((clock, HashMap::new())),
+            idle_timeout: self.idle_timeout,
             distributions: RwLock::new(HashMap::new()),
             quantiles: self.quantiles.clone(),
             buckets: self.buckets.clone(),
@@ -496,12 +567,14 @@ impl PrometheusBuilder {
     ) -> Result<
         (
             PrometheusRecorder,
-            impl Future<Output = Result<(), HyperError>> + Send + Sync + 'static,
+            impl Future<Output = Result<(), HyperError>> + Send + 'static,
         ),
         Error,
     > {
         let inner = Arc::new(Inner {
             registry: Registry::new(),
+            recency: Mutex::new((Clock::new(), HashMap::new())),
+            idle_timeout: self.idle_timeout,
             distributions: RwLock::new(HashMap::new()),
             quantiles: self.quantiles.clone(),
             buckets: self.buckets.clone(),
@@ -593,15 +666,10 @@ impl Recorder for PrometheusRecorder {
 }
 
 fn key_to_parts(key: Key) -> (String, Vec<String>) {
-    let name = key.name();
-    let labels = key.labels();
     let sanitize = |c| c == '.' || c == '=' || c == '{' || c == '}' || c == '+' || c == '-';
-    let name = name
-        .parts()
-        .map(|s| s.replace(sanitize, "_"))
-        .collect::<Vec<_>>()
-        .join("_");
-    let labels = labels
+    let name = key.name().to_string().replace(sanitize, "_");
+    let labels = key
+        .labels()
         .into_iter()
         .map(|label| {
             let k = label.key();
diff --git a/metrics-util/src/bucket.rs b/metrics-util/src/bucket.rs
index e3fc6ae..372a80c 100644
--- a/metrics-util/src/bucket.rs
+++ b/metrics-util/src/bucket.rs
@@ -38,6 +38,17 @@ impl<T> Block<T> {
         }
     }
 
+    // Gets the length of the previous block, if it exists.
+    pub(crate) fn prev_len(&self, guard: &Guard) -> usize {
+        let tail = self.prev.load(Ordering::Acquire, guard);
+        if tail.is_null() {
+            return 0;
+        }
+
+        let tail_block = unsafe { tail.deref() };
+        tail_block.len()
+    }
+
     /// Gets the current length of this block.
     pub fn len(&self) -> usize {
         self.read.load(Ordering::Acquire).trailing_ones() as usize
@@ -123,6 +134,18 @@ impl<T> AtomicBucket<T> {
         }
     }
 
+    /// Checks whether or not this bucket is empty.
+    pub fn is_empty(&self) -> bool {
+        let guard = &epoch_pin();
+        let tail = self.tail.load(Ordering::Acquire, guard);
+        if tail.is_null() {
+            return true;
+        }
+
+        let tail_block = unsafe { tail.deref() };
+        tail_block.len() == 0 && tail_block.prev_len(&guard) == 0
+    }
+
     /// Pushes an element into the bucket.
     pub fn push(&self, value: T) {
         let mut original = value;
@@ -521,4 +544,23 @@ mod tests {
         let snapshot = bucket.data();
         assert_eq!(snapshot.len(), 0);
     }
+
+    #[test]
+    fn test_bucket_len_and_prev_len() {
+        let bucket = AtomicBucket::new();
+        assert!(bucket.is_empty());
+
+        let snapshot = bucket.data();
+        assert_eq!(snapshot.len(), 0);
+
+        // Just making sure that `is_empty` holds as we go from
+        // the first block, to the second block, to exercise the
+        // `Block::prev_len` codepath.
+        let mut i = 0;
+        while i < BLOCK_SIZE * 2 {
+            bucket.push(i);
+            assert!(!bucket.is_empty());
+            i += 1;
+        }
+    }
 }
diff --git a/metrics-util/src/debugging.rs b/metrics-util/src/debugging.rs
index 4c6ab66..2f88fa2 100644
--- a/metrics-util/src/debugging.rs
+++ b/metrics-util/src/debugging.rs
@@ -116,13 +116,13 @@ impl Snapshotter {
                 };
 
                 for (dk, _) in metrics.into_iter() {
-                    if let Some(h) = handles.get(&dk) {
+                    if let Some((_, h)) = handles.get(&dk) {
                         collect_metric(dk, h, &self.units, &self.descs, &mut snapshot);
                     }
                 }
             }
             None => {
-                for (dk, h) in handles.into_iter() {
+                for (dk, (_, h)) in handles.into_iter() {
                     collect_metric(dk, &h, &self.units, &self.descs, &mut snapshot);
                 }
             }
diff --git a/metrics-util/src/handle.rs b/metrics-util/src/handle.rs
index 2aad114..2c9e8b4 100644
--- a/metrics-util/src/handle.rs
+++ b/metrics-util/src/handle.rs
@@ -107,6 +107,16 @@ impl Handle {
         }
     }
 
+    /// Reads this handle as a histogram, and whether or not it's empty.
+    ///
+    /// Panics if this handle is not a histogram.
+    pub fn read_histogram_is_empty(&self) -> bool {
+        match self {
+            Handle::Histogram(bucket) => bucket.is_empty(),
+            _ => panic!("tried to read as histogram"),
+        }
+    }
+
     /// Reads this handle as a histogram incrementally into a closure, and clears the histogram.
     ///
     /// The closure `f` passed in is invoked multiple times with slices of values present in the
diff --git a/metrics-util/src/registry.rs b/metrics-util/src/registry.rs
index ebbee01..ec81341 100644
--- a/metrics-util/src/registry.rs
+++ b/metrics-util/src/registry.rs
@@ -1,7 +1,35 @@
-use core::hash::Hash;
+use core::{
+    hash::Hash,
+    sync::atomic::{AtomicUsize, Ordering},
+};
 use dashmap::DashMap;
 use std::collections::HashMap;
 
+#[derive(Debug)]
+struct Generational<H>(AtomicUsize, H);
+
+impl<H: Clone> Generational<H> {
+    pub fn new(h: H) -> Generational<H> {
+        Generational(AtomicUsize::new(0), h)
+    }
+
+    pub fn increment_generation(&self) {
+        self.0.fetch_add(1, Ordering::Release);
+    }
+
+    pub fn get_generation(&self) -> usize {
+        self.0.load(Ordering::Acquire)
+    }
+
+    pub fn get_inner(&self) -> &H {
+        &self.1
+    }
+
+    pub fn to_owned(&self) -> (usize, H) {
+        (self.get_generation(), self.get_inner().clone())
+    }
+}
+
 /// A high-performance metric registry.
 ///
 /// `Registry` provides the ability to maintain a central listing of metrics mapped by a given key.
@@ -14,17 +42,20 @@ use std::collections::HashMap;
 /// update the actual metric value(s) as needed.  `Handle`, from this crate, is a solid default
 /// choice.
 ///
-/// `Registry` handles deduplicating metrics, and will return the `Identifier` for an existing
-/// metric if a caller attempts to reregister it.
+/// As well, handles have an associated generation counter which is incremented any time an entry is
+/// operated on.  This generation is returned with the handle when querying the registry, and can be
+/// used in order to delete a handle from the registry, allowing callers to prune old/stale handles
+/// over time.
 ///
 /// `Registry` is optimized for reads.
 pub struct Registry<K, H> {
-    map: DashMap<K, H>,
+    map: DashMap<K, Generational<H>>,
 }
 
 impl<K, H> Registry<K, H>
 where
-    K: Eq + Hash + Clone,
+    K: Eq + Hash + Clone + 'static,
+    H: Clone + 'static,
 {
     /// Creates a new `Registry`.
     pub fn new() -> Self {
@@ -44,23 +75,34 @@ where
         I: FnOnce() -> H,
         O: FnOnce(&H) -> V,
     {
-        let valref = self.map.entry(key).or_insert_with(init);
-        op(valref.value())
+        let valref = self.map.entry(key).or_insert_with(|| {
+            let value = init();
+            Generational::new(value)
+        });
+        let value = valref.value();
+        let result = op(value.get_inner());
+        value.increment_generation();
+        result
+    }
+
+    /// Deletes a handle from the registry.
+    ///
+    /// The generation of a given key is passed along when querying the registry via
+    /// [`get_handles`].  If the generation given here does not match the current generation, then
+    /// the handle will not be removed.
+    pub fn delete(&self, key: &K, generation: usize) -> bool {
+        self.map
+            .remove_if(key, |_, g| g.get_generation() == generation)
+            .is_some()
     }
-}
 
-impl<K, H> Registry<K, H>
-where
-    K: Eq + Hash + Clone + 'static,
-    H: Clone + 'static,
-{
     /// Gets a map of all present handles, mapped by key.
     ///
     /// Handles must implement `Clone`.  This map is a point-in-time snapshot of the registry.
-    pub fn get_handles(&self) -> HashMap<K, H> {
+    pub fn get_handles(&self) -> HashMap<K, (usize, H)> {
         self.map
             .iter()
-            .map(|item| (item.key().clone(), item.value().clone()))
+            .map(|item| (item.key().clone(), item.value().to_owned()))
             .collect()
     }
 }