diff --git a/src/combat.lua b/src/combat.lua
new file mode 100644
index 0000000..beca537
--- /dev/null
+++ b/src/combat.lua
@@ -0,0 +1,26 @@
+-- Deterministic combat: damage is effective attack minus effective defense,
+-- floored at 1. Effective stats fold in temporary status modifiers.
+
+local Combat = {}
+
+local function attackOf(entity)
+  return entity.effectiveAttack and entity:effectiveAttack() or entity.attack
+end
+
+local function defenseOf(entity)
+  return entity.effectiveDefense and entity:effectiveDefense() or entity.defense
+end
+
+function Combat.damage(attacker, defender)
+  local dealt = attackOf(attacker) - defenseOf(defender)
+  if dealt < 1 then dealt = 1 end
+  return dealt
+end
+
+function Combat.attack(attacker, defender)
+  local dealt = Combat.damage(attacker, defender)
+  defender.hp = defender.hp - dealt
+  return { damage = dealt, killed = defender.hp <= 0 }
+end
+
+return Combat