diff --git a/src/lib.rs b/src/lib.rs
index 4cd242a..417e3ba 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -84,7 +84,7 @@ impl fmt::Display for Card {
 }
 
 #[repr(u8)]
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq, Debug)]
 pub enum Action {
     Income,
     ForeignAid,
@@ -161,6 +161,9 @@ impl Player {
         self.cards.draw_first(card);
         deck.push(card);
     }
+    pub fn holds(&self, card: Card) -> bool {
+        self.cards.iter().find(|&c| *c == card).is_some()
+    }
 }
 
 impl Default for Player {
@@ -219,7 +222,43 @@ impl Game {
         players_turn_order.skip(1).filter(|p| self.players[*p].is_alive())
     }
 
-    fn resolution(&mut self, resolution: Resolution, agents: &[&dyn Agent]) -> CoupResult<Phase> {
+    fn advance(&mut self) {
+        let next = self.turn_iterator().next();
+        if let Some(next) = next {
+            self.turn = next;
+        }
+    }
+
+    pub fn block_challenge(&mut self, block_challenge: BlockChallenge, agents: &[&dyn Agent]) -> CoupResult<Phase> {
+        if agents[self.turn].should_block_challenge(&block_challenge) {
+            if self.players[block_challenge.blocker].holds(block_challenge.block_card) {
+                // Player challenged incorrectly, loses influence and turn is forfeit
+                let current_player = &mut self.players[self.turn];
+                let card = agents[self.turn]
+                    .choose_lost_influence(&current_player.cards);
+                current_player.lose(card, &mut self.discard);
+                Ok(Phase::Done)
+            } else {
+                // Player challenged correctly, blocker loses a card
+                let blocking_player = &mut self.players[block_challenge.blocker];
+                let card = agents[block_challenge.blocker]
+                    .choose_lost_influence(&blocking_player.cards);
+                blocking_player.lose(card, &mut self.discard);
+
+                // Game continues
+                Ok(Phase::Resolution(Resolution {
+                    action: block_challenge.action,
+                    target: block_challenge.target,
+                }))
+            }
+        } else {
+            // Player chose not to challenge the block, turn is forfeit
+            self.advance();
+            Ok(Phase::Done)
+        }   
+    }
+
+    pub fn resolution(&mut self, resolution: Resolution, agents: &[&dyn Agent]) -> CoupResult<Phase> {
         let current_player = &mut self.players.get_mut(self.turn).unwrap();
         let current_agent = agents[self.turn];
         match resolution.action {
@@ -228,8 +267,10 @@ impl Game {
             Coup | Assassinate => match resolution.target {
                 Some(target) => {
                     let target_player = &self.players[target];
-                    let card = agents[target].choose_lost_influence(&target_player.cards);
-                    self.players[target].lose(card, &mut self.discard);
+                    if target_player.is_alive() { // Target may have died from challenge
+                        let card = agents[target].choose_lost_influence(&target_player.cards);
+                        self.players[target].lose(card, &mut self.discard);
+                    }
                 }
                 _ => return Err("Coup/Assassinate resolution has no target"),
             },
@@ -259,29 +300,37 @@ impl Game {
                 _ => return Err("Steal resolution has no target"),
             }        
         }
-        let next = self.turn_iterator().next();
-        if let Some(next) = next {
-            self.turn = next;
-        }
+        self.advance();
         Ok(Phase::Done)
     }
 }
 
-enum Phase {
+#[derive(PartialEq, Debug)]
+pub enum Phase {
     Action(Action),
     //ActionChallenge(ActionChallenge),
     //Block(Block),
-    //BlockChallenge(BlockChallenge),
+    BlockChallenge(BlockChallenge),
     Resolution(Resolution),
     Done,
 }
 
+#[derive(PartialEq, Debug)]
+pub struct BlockChallenge {
+    blocker: usize,
+    block_card: Card,
+    action: Action,
+    target: Option<usize>,
+}
+
+#[derive(PartialEq, Debug)]
 pub struct Resolution {
     action: Action,
     target: Option<usize>,
 }
 
 pub trait Agent : fmt::Debug {
+    fn should_block_challenge(&self, block_challenge: &BlockChallenge) -> bool;
     fn exchange(&self, cards: &[Card]) -> [Card; 2];
     fn choose_lost_influence(&self, cards: &[Card]) -> Card;
 }
@@ -290,18 +339,102 @@ pub trait Agent : fmt::Debug {
 mod test {
     use super::*;
 
+    macro_rules! make_dummy_agent {
+        ($sbc: expr, $e: expr, $cli: expr) => {{
+            #[derive(Debug)]
+            struct DummyAgent;
+            impl Agent for DummyAgent {
+                #[allow(unused)]
+                fn should_block_challenge(&self, block_challenge: &BlockChallenge) -> bool {
+                    $sbc
+                }
+                #[allow(unused)]
+                fn exchange(&self, cards: &[Card]) -> [Card; 2] {
+                    $e
+                }
+                #[allow(unused)]
+                fn choose_lost_influence(&self, cards: &[Card]) -> Card {
+                    $cli
+                }
+            }
+            DummyAgent
+        }}
+    }
+
+    #[test]
+    fn test_block_challenge() {
+        let challenge_agent = make_dummy_agent!(true, unimplemented!(), Assassin);
+        let non_challenge_agent = make_dummy_agent!(false, unimplemented!(), Assassin);
+        let block_agent = make_dummy_agent!(unimplemented!(), unimplemented!(), Contessa);
+        let deck = vec![];
+        let discard = vec![];
+        let players = vec![
+            Player { coins: 2, cards: vec![Assassin] },
+            Player { coins: 2, cards: vec![Contessa] },
+        ];
+        let game = Game {
+            deck,
+            discard,
+            players,
+            turn: 0
+        };
+
+        // Test non-challenge
+        {
+            let mut game = game.clone();
+            assert_eq!(
+                game.block_challenge( BlockChallenge {
+                    blocker: 1,
+                    block_card: Contessa,
+                    action: Assassinate,
+                    target: Some(1),
+                }, &[&non_challenge_agent, &block_agent]),
+                Ok(Phase::Done)
+            );
+            assert!(game.discard.is_empty());
+        }
+
+        // Test failed challenge
+        {
+            let mut game = game.clone();
+            assert_eq!(
+                game.block_challenge( BlockChallenge {
+                    blocker: 1,
+                    block_card: Contessa,
+                    action: Assassinate,
+                    target: Some(1),
+                }, &[&challenge_agent, &block_agent]),
+                Ok(Phase::Done)
+            );
+            assert!(!game.discard.is_empty());
+        }
+
+          // Test successful challenge
+        {
+            let mut game = game.clone();
+            assert_eq!(
+                game.block_challenge( BlockChallenge {
+                    blocker: 1,
+                    block_card: Duke,
+                    action: ForeignAid,
+                    target: None,
+                }, &[&challenge_agent, &block_agent]),
+                Ok(Phase::Resolution(Resolution {
+                    action: ForeignAid,
+                    target: None,
+                }))
+            );
+            assert!(!game.discard.is_empty());
+        }
+        
+    }
+        
+            
+        
+
     #[test]
     fn test_resolution() {
-        #[derive(Debug)]
-        struct DummyAgent;
-        impl Agent for DummyAgent {
-            fn exchange(&self, cards: &[Card]) -> [Card; 2] {
-                [Contessa, Duke]
-            }
-            fn choose_lost_influence(&self, cards: &[Card]) -> Card {
-                Captain
-            }
-        }
+        let dummy_agent = make_dummy_agent!(unimplemented!(), [Contessa, Duke], Captain);
         let deck = vec![Contessa, Contessa];
         let discard = vec![];
         let players = vec![
@@ -321,7 +454,7 @@ mod test {
             game.resolution(Resolution {
                 action: Income,
                 target: None,
-            }, &[&DummyAgent, &DummyAgent]).unwrap();
+            }, &[&dummy_agent, &dummy_agent]).unwrap();
             assert_eq!(game.players[0].coins, 3);
         }
 
@@ -331,7 +464,7 @@ mod test {
             game.resolution(Resolution {
                 action: ForeignAid,
                 target: None,
-            }, &[&DummyAgent, &DummyAgent]).unwrap();
+            }, &[&dummy_agent, &dummy_agent]).unwrap();
             assert_eq!(game.players[0].coins, 4);
         }
 
@@ -341,7 +474,7 @@ mod test {
             game.resolution(Resolution {
                 action: Coup,
                 target: Some(1),
-            }, &[&DummyAgent, &DummyAgent]).unwrap();
+            }, &[&dummy_agent, &dummy_agent]).unwrap();
             assert!(game.players[1].cards.is_empty());
             assert_eq!(game.discard, vec![Captain]);
         }
@@ -352,7 +485,7 @@ mod test {
             game.resolution(Resolution {
                 action: Steal,
                 target: Some(1),
-            }, &[&DummyAgent, &DummyAgent]).unwrap();
+            }, &[&dummy_agent, &dummy_agent]).unwrap();
             assert_eq!(game.players[0].coins, 3);
             assert_eq!(game.players[1].coins, 0);
         }
@@ -363,7 +496,7 @@ mod test {
             game.resolution(Resolution {
                 action: Exchange,
                 target: Some(1),
-            }, &[&DummyAgent, &DummyAgent]).unwrap();
+            }, &[&dummy_agent, &dummy_agent]).unwrap();
             game.players[0].cards.sort();
             assert_eq!(game.players[0].cards, vec![Assassin, Contessa]);
             game.deck.sort();