diff --git a/tests/net/mod.rs b/tests/net/mod.rs
index 7ac233c..5748ea7 100644
--- a/tests/net/mod.rs
+++ b/tests/net/mod.rs
@@ -22,7 +22,7 @@ use std::{cmp, collections, env, fmt, fs, io, ops, process, time};
 
 use rand;
 use rand::{Rand, Rng};
-use threshold_crypto as crypto;
+use threshold_crypto::{self as crypto, SecretKeyShare};
 
 use hbbft::dynamic_honey_badger::Batch;
 use hbbft::util::SubRng;
@@ -302,7 +302,7 @@ pub struct NetBuilder<D, I>
 where
     D: DistAlgorithm,
 {
-    /// Iterator used to create node ids.
+    /// Iterator used to create validator node ids.
     node_ids: I,
     /// Number of faulty nodes in the network.
     num_faulty: usize,
@@ -318,6 +318,9 @@ where
     message_limit: Option<usize>,
     /// Optional time limit.
     time_limit: Option<time::Duration>,
+    /// Optional observer node ID. It must not be a member of `node_ids` for the latter are
+    /// validator node IDs.
+    observer_id: Option<D::NodeId>,
     /// Random number generator used to generate keys.
     rng: Option<Box<dyn Rng>>,
 }
@@ -336,6 +339,7 @@ where
             .field("crank_limit", &self.crank_limit)
             .field("message_limit", &self.message_limit)
             .field("time_limit", &self.time_limit)
+            .field("observer_id", &self.observer_id)
             .field("rng", &"<RNG>")
             .finish()
     }
@@ -368,6 +372,7 @@ where
             message_limit: None,
             time_limit: DEFAULT_TIME_LIMIT,
             rng: None,
+            observer_id: None,
         }
     }
 
@@ -456,6 +461,15 @@ where
         self
     }
 
+    /// Override the default observer setting.
+    ///
+    /// Adds an additional, observer node to the network.
+    #[inline]
+    pub fn observer_id(mut self, observer_id: D::NodeId) -> Self {
+        self.observer_id = Some(observer_id);
+        self
+    }
+
     /// Constructor function (with step).
     ///
     /// The constructor function is used to construct each node in the network. Any step returned
@@ -513,9 +527,13 @@ where
 
         // Note: Closure is not redundant, won't compile without it.
         #[cfg_attr(feature = "cargo-clippy", allow(redundant_closure))]
-        let mut net = VirtualNet::new(self.node_ids, self.num_faulty as usize, rng, move |node| {
-            cons(node)
-        })?;
+        let mut net = VirtualNet::new(
+            self.node_ids,
+            self.num_faulty as usize,
+            rng,
+            self.observer_id,
+            move |node| cons(node),
+        )?;
 
         if self.adversary.is_some() {
             net.adversary = self.adversary;
@@ -569,6 +587,8 @@ where
     time_limit: Option<time::Duration>,
     /// The instant the network was created.
     start_time: time::Instant,
+    /// Optional observer node ID.
+    observer_id: Option<D::NodeId>,
 }
 
 impl<D> fmt::Debug for VirtualNet<D>
@@ -700,6 +720,11 @@ where
         msgs.sort_by(f);
         self.messages.extend(msgs);
     }
+
+    /// Returns a reference to the observer node ID if it is defined and `None` otherwise.
+    pub fn observer_id(&self) -> Option<&D::NodeId> {
+        self.observer_id.as_ref()
+    }
 }
 
 impl<D> VirtualNet<D>
@@ -726,6 +751,7 @@ where
         node_ids: I,
         faulty: usize,
         mut rng: R,
+        observer_id: Option<D::NodeId>,
         cons: F,
     ) -> Result<Self, crypto::error::Error>
     where
@@ -734,22 +760,29 @@ where
         R: rand::Rng,
     {
         // Generate a new set of cryptographic keys for threshold cryptography.
-        let net_infos = NetworkInfo::generate_map(node_ids, &mut rng)?;
+        let netinfos = NetworkInfo::generate_map(node_ids, &mut rng)?;
 
         assert!(
-            faulty * 3 < net_infos.len(),
+            faulty * 3 < netinfos.len(),
             "Too many faulty nodes requested, `f` must satisfy `3f < total_nodes`."
         );
-
+        let observer_netinfo = observer_id.map(|observer_id| {
+            let node_ni = netinfos.values().next().unwrap();
+            NetworkInfo::new(
+                observer_id,
+                SecretKeyShare::default(),
+                node_ni.public_key_set().clone(),
+                rng.gen(),
+                node_ni.public_key_map().clone(),
+            )
+        });
         let mut steps = collections::BTreeMap::new();
         let mut messages = collections::VecDeque::new();
-
-        let mut nodes = net_infos
+        let mut nodes: collections::BTreeMap<_, _> = netinfos
             .into_iter()
             .enumerate()
             .map(|(idx, (id, netinfo))| {
                 let is_faulty = idx < faulty;
-
                 let (algorithm, step) = cons(NewNodeInfo {
                     id: id.clone(),
                     netinfo,
@@ -759,7 +792,19 @@ where
                 steps.insert(id.clone(), step);
                 (id, Node::new(algorithm, is_faulty))
             }).collect();
-
+        // Add the observer node if required.
+        let observer_id = observer_netinfo.map(|netinfo| {
+            let id = netinfo.our_id().clone();
+            let (algorithm, step) = cons(NewNodeInfo {
+                id: id.clone(),
+                netinfo,
+                faulty: false,
+                rng: rng.sub_rng(),
+            });
+            steps.insert(id.clone(), step);
+            nodes.insert(id.clone(), Node::new(algorithm, false));
+            id
+        });
         let mut message_count: usize = 0;
         // For every recorded step, apply it.
         for (sender, step) in steps {
@@ -782,6 +827,7 @@ where
             message_limit: None,
             time_limit: None,
             start_time: time::Instant::now(),
+            observer_id,
         })
     }