diff --git a/src/consumer_groups/register.rs b/src/consumer_groups/register.rs
index d897e4b..9c4413d 100644
--- a/src/consumer_groups/register.rs
+++ b/src/consumer_groups/register.rs
@@ -274,5 +274,222 @@ impl Awaitable for ConsumerGroupsRegister {
 
 #[cfg(test)]
 mod test {
-    // TODO...
+    use std::{
+        collections::{HashMap, HashSet},
+        future::Future,
+        sync::Arc,
+    };
+
+    use prometheus::Registry;
+    use tokio::{
+        sync::mpsc,
+        time::{sleep, Duration, Instant},
+    };
+
+    use crate::consumer_groups::{emitter::ConsumerGroups, ConsumerGroupsRegister};
+    use crate::kafka_types::{Group, GroupWithMembers, Member, MemberWithAssignment};
+
+    fn make_group_with_members(
+        g_name: &str,
+        m_prefix: &str,
+        count: usize,
+    ) -> (String, GroupWithMembers) {
+        let mut gwm = GroupWithMembers {
+            group: Group {
+                name: format!("{}", g_name),
+                state: format!("{}-state", g_name),
+                protocol: format!("{}-protocol", g_name),
+                protocol_type: format!("{}-protocol_type", g_name),
+            },
+            members: HashMap::with_capacity(count),
+        };
+
+        for i in 0..count {
+            let m_id = format!("{}{}{}", g_name, m_prefix, i);
+            gwm.members.insert(
+                m_id.clone(),
+                MemberWithAssignment {
+                    member: Member {
+                        client_id: format!("{}-client_id", m_id),
+                        client_host: format!("{}-client_host", m_id),
+                        id: m_id,
+                    },
+                    assignment: HashSet::new(),
+                },
+            );
+        }
+
+        (g_name.to_string(), gwm)
+    }
+
+    const BLOCK_ON_CONDITION_CHECK_FREQ: Duration = Duration::from_millis(10);
+
+    async fn block_on<F, Fut>(f: F, timeout: Duration)
+    where
+        F: Fn() -> Fut,
+        Fut: Future<Output = bool>,
+    {
+        let start = Instant::now();
+        loop {
+            if f().await {
+                break;
+            }
+
+            assert!(
+                Instant::now().duration_since(start) < timeout,
+                "Timed out waiting on desired condition"
+            );
+            sleep(BLOCK_ON_CONDITION_CHECK_FREQ).await;
+        }
+    }
+
+    #[tokio::test(flavor = "multi_thread")]
+    async fn should_forget_after() {
+        let (sx, rx) = mpsc::channel::<ConsumerGroups>(10);
+        let forget_after = Duration::from_secs(3);
+        let metrics_reg_arc = Arc::new(Registry::new());
+
+        let reg_under_test = ConsumerGroupsRegister::new(rx, forget_after, metrics_reg_arc);
+
+        // Start with 3 groups
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([
+                make_group_with_members("g1", "m", 10),
+                make_group_with_members("g2", "m", 5),
+                make_group_with_members("g3", "m", 2),
+            ]),
+        })
+        .await
+        .unwrap();
+
+        // Wait for 3 groups to be visible when querying Register
+        block_on(|| async { reg_under_test.get_groups_count().await == 3 }, Duration::from_secs(5))
+            .await;
+        assert!(reg_under_test.contains_group("g1").await);
+        assert!(reg_under_test.contains_group("g2").await);
+        assert!(reg_under_test.contains_group("g3").await);
+
+        // A new group is detected
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([
+                make_group_with_members("g1", "m", 10),
+                make_group_with_members("g2", "m", 5),
+                make_group_with_members("g3", "m", 2),
+                make_group_with_members("g4", "m", 15),
+            ]),
+        })
+        .await
+        .unwrap();
+
+        // Confirm new group is visible
+        block_on(|| async { reg_under_test.get_groups_count().await == 4 }, Duration::from_secs(5))
+            .await;
+        assert!(reg_under_test.contains_group("g4").await);
+
+        // Wait 2 seconds, then remove 2 groups
+        sleep(Duration::from_secs(2)).await;
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([
+                make_group_with_members("g1", "m", 10),
+                make_group_with_members("g3", "m", 2),
+            ]),
+        })
+        .await
+        .unwrap();
+
+        // Confirm all 4 groups are still visible, for a little while longer
+        block_on(|| async { reg_under_test.get_groups_count().await == 4 }, Duration::from_secs(5))
+            .await;
+        assert!(reg_under_test.contains_group("g1").await);
+        assert!(reg_under_test.contains_group("g2").await);
+        assert!(reg_under_test.contains_group("g3").await);
+        assert!(reg_under_test.contains_group("g4").await);
+
+        // Confirm that, 2 groups removed earlier are now finally gone
+        block_on(
+            || async { reg_under_test.get_groups_count().await == 2 },
+            Duration::from_secs(20),
+        )
+        .await;
+        assert!(reg_under_test.contains_group("g1").await);
+        assert!(reg_under_test.contains_group("g3").await);
+    }
+
+    #[tokio::test(flavor = "multi_thread")]
+    async fn should_track_hash() {
+        let (sx, rx) = mpsc::channel::<ConsumerGroups>(10);
+        let forget_after = Duration::from_secs(3);
+        let metrics_reg_arc = Arc::new(Registry::new());
+
+        let reg = ConsumerGroupsRegister::new(rx, forget_after, metrics_reg_arc);
+
+        // Start with no groups and grab the "empty" hash
+        sx.send(ConsumerGroups {
+            groups: HashMap::new(),
+        })
+        .await
+        .unwrap();
+        let initial_hash = reg.get_hash().await;
+        let mut curr_hash = initial_hash;
+
+        // Start with 1 group
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([make_group_with_members("g1", "m", 10)]),
+        })
+        .await
+        .unwrap();
+        block_on(|| async { reg.get_groups_count().await == 1 }, Duration::from_secs(5)).await;
+        let mut new_hash = reg.get_hash().await;
+        assert_ne!(curr_hash, new_hash);
+        curr_hash = new_hash;
+
+        // Unchanged
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([make_group_with_members("g1", "m", 10)]),
+        })
+        .await
+        .unwrap();
+        block_on(|| async { reg.get_groups_count().await == 1 }, Duration::from_secs(5)).await;
+        new_hash = reg.get_hash().await;
+        assert_eq!(curr_hash, new_hash);
+
+        // Add a group
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([
+                make_group_with_members("g1", "m", 10),
+                make_group_with_members("g2", "m", 20),
+            ]),
+        })
+        .await
+        .unwrap();
+        block_on(|| async { reg.get_groups_count().await == 2 }, Duration::from_secs(5)).await;
+        new_hash = reg.get_hash().await;
+        assert_ne!(curr_hash, new_hash);
+        curr_hash = new_hash;
+
+        // Wait 2 seconds, then remove 1 groups
+        sleep(Duration::from_secs(2)).await;
+        sx.send(ConsumerGroups {
+            groups: HashMap::from([make_group_with_members("g2", "m", 20)]),
+        })
+        .await
+        .unwrap();
+
+        // Confirm 2 groups are still visible, for a little while longer, and hash hasn't yet changed
+        block_on(|| async { reg.get_groups_count().await == 2 }, Duration::from_secs(5)).await;
+        new_hash = reg.get_hash().await;
+        assert_eq!(curr_hash, new_hash);
+
+        // Confirm that, eventually, only 1 group is left and hash gets updated
+        block_on(|| async { reg.get_groups_count().await == 1 }, Duration::from_secs(20)).await;
+        new_hash = reg.get_hash().await;
+        assert_ne!(curr_hash, new_hash);
+        curr_hash = new_hash;
+
+        // Confirm that, eventually, all groups are removed and the hash goes back to the "empty" one
+        block_on(|| async { reg.get_groups_count().await == 0 }, Duration::from_secs(20)).await;
+        new_hash = reg.get_hash().await;
+        assert_ne!(curr_hash, new_hash);
+        assert_eq!(new_hash, initial_hash);
+    }
 }