diff --git a/README.md b/README.md
index 7aed2b1..e497263 100644
--- a/README.md
+++ b/README.md
@@ -331,10 +331,12 @@ Règles modélisées :
   l'arme ranged reste utilisable jusqu'à 10 incréments. Les armes de jet
   (5 incréments max) ne sont pas distinguées des armes à projectiles.
 
-Non modélisé en Phase 0 (couches `elevation`/`markers` présentes mais non
-appliquées dans la résolution) :
+Non modélisé (couches `elevation`/`markers` présentes mais non appliquées
+dans la résolution) :
 
-- Sorts, jets de sauvegarde, conditions et états.
+- Sorts, jets de sauvegarde, conditions et états (le système d'effets et les
+  règles d'empilement sont en place via `effects.py` — l'intégration au
+  moteur est en cours).
 - Manœuvres de combat.
 - Effets mécaniques de hauteur/élévation.
 - Tailles Large+ (2×2), allonge > 5 ft
@@ -357,6 +359,12 @@ appliquées dans la résolution) :
 - `combat.py` — `CombatEngine` déterministe : initiative, actions, résolution
   des attaques (couvert, ligne d'effet, pénalités de portée), états de vie,
   transcripts, politique par défaut.
+- `effects.py` — système d'effets : `StatModifier` (modificateur de
+  caractéristique avec type de bonus) et `resolve_modifiers` (application des
+  règles d'empilement PF1e). Types de bonus cumulables (dodge, racial, trait,
+  sans type) s'additionnent ; types non-cumulables (morale, sacré, profane,
+  enhancement…) gardent la valeur la plus élevée. Les pénalités suivent les
+  mêmes règles. Fondation pour sorts, conditions, dons et capacités de classe.
 - `metrics.py` — statistiques en forme fermée : `win_rate`, `win_rate_sigma`,
   `win_rate_band` (bande 3σ bornée à [0, 1]).
 - `runner.py` — `EncounterSpec`/`Side`, `build_states` (placement en zone +
@@ -372,7 +380,7 @@ appliquées dans la résolution) :
 La gate de validation complète (tests + lint + types) :
 
 ```bash
-uv run pytest -q          # 219 tests
+uv run pytest -q          # 241 tests
 uv run ruff check src tests
 uv run basedpyright src   # mode strict
 ```
@@ -386,9 +394,10 @@ uv run basedpyright src   # mode strict
 ## Feuille de route
 
 - **Phase 1** — magie et états : jets de sauvegarde, sorts modélisés comme
-  effets paramétrés, conditions, manœuvres de combat. Flanquement, attaques à
-  outrance, attaques d'opportunité, charge, retraite et pas de placement sont
-  déjà modélisés.
+  effets paramétrés, conditions, manœuvres de combat, dons et capacités de
+  classe. Flanquement, attaques à outrance, attaques d'opportunité, charge,
+  retraite et pas de placement sont déjà modélisés. Le système d'effets
+  (`effects.py`) et les règles d'empilement des bonus sont en place.
 - **Phase 2** — couche tactique LLM : stratégies en langage naturel traduites
   en politiques, balayage de matrices de positionnement.
 - **Phase 3** — rapporteur LLM local : agrégation des statistiques et
diff --git a/src/pf1e_simulator/effects.py b/src/pf1e_simulator/effects.py
new file mode 100644
index 0000000..8fa4f52
--- /dev/null
+++ b/src/pf1e_simulator/effects.py
@@ -0,0 +1,88 @@
+"""Effect system: bonus types, stat modifiers, and PF1e stacking rules.
+
+PF1e bonus types determine whether multiple bonuses to the same stat stack:
+- Stacking types (dodge, racial, trait, untyped/"") all sum together.
+- Non-stacking types (morale, sacred, profane, competence, insight, luck,
+  enhancement, deflection, natural) keep only the highest value per type.
+- Penalties follow the same rules: same-type penalties don't stack (the
+  least severe — i.e. the max value — is kept).
+
+This module is the foundation for spells, conditions, feats, class abilities,
+and racial traits — all of which produce StatModifier instances that the
+engine collects and sums via resolve_modifiers.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Literal
+
+if TYPE_CHECKING:
+    from collections.abc import Sequence
+
+BonusType = Literal[
+    "",               # untyped — stacks with everything
+    "dodge",          # stacks with itself
+    "morale",         # no stack
+    "sacred",         # no stack
+    "profane",        # no stack
+    "competence",     # no stack
+    "insight",        # no stack
+    "luck",           # no stack
+    "racial",         # stacks with itself
+    "trait",          # stacks with itself
+    "enhancement",    # no stack
+    "deflection",     # no stack
+    "natural",        # no stack
+    "circumstance",   # no stack (simulator can't distinguish circumstances)
+]
+
+StatTarget = Literal[
+    "attack", "damage", "ac", "touch_ac", "flat_footed_ac",
+    "fort", "ref", "will", "initiative",
+    "cmb", "cmd", "speed_ft", "hp", "concentration", "cl",
+]
+
+_STACKING_TYPES: frozenset[str] = frozenset({"", "dodge", "racial", "trait"})
+
+
+@dataclass(frozen=True)
+class StatModifier:
+    """+X to a stat, optionally with a bonus type for stacking and a condition.
+
+    - ``bonus_type`` defaults to "" (untyped) which stacks with everything.
+    - ``condition`` is None for always-active modifiers; otherwise it names a
+      runtime condition (e.g. "target_within_30ft", "flanking") that the engine
+      evaluates before including this modifier.
+    - ``weapon_filter`` restricts the modifier to a specific weapon by name
+      (e.g. "Pistol" for Weapon Focus (Pistol)).
+    """
+
+    target: StatTarget
+    value: int
+    bonus_type: BonusType = ""
+    condition: str | None = None
+    weapon_filter: str | None = None
+
+
+def resolve_modifiers(modifiers: Sequence[StatModifier]) -> int:
+    """Sum stat modifiers applying PF1e stacking rules.
+
+    Stacking bonus types (dodge, racial, trait, untyped) all sum together.
+    Non-stacking types keep only the highest value per type (max is used,
+    which correctly handles both bonuses and penalties: for bonuses the
+    highest is most beneficial, for penalties the highest is least severe).
+
+    The caller is responsible for filtering by condition and weapon before
+    calling — this function performs pure stacking math.
+    """
+    stacking_total = 0
+    best_by_type: dict[str, int] = {}
+    for mod in modifiers:
+        if mod.bonus_type in _STACKING_TYPES:
+            stacking_total += mod.value
+        else:
+            prev = best_by_type.get(mod.bonus_type)
+            if prev is None or mod.value > prev:
+                best_by_type[mod.bonus_type] = mod.value
+    return stacking_total + sum(best_by_type.values())
diff --git a/tests/test_effects.py b/tests/test_effects.py
new file mode 100644
index 0000000..0cac811
--- /dev/null
+++ b/tests/test_effects.py
@@ -0,0 +1,204 @@
+"""Tests for the effect system: StatModifier and PF1e bonus stacking rules."""
+
+from __future__ import annotations
+
+from pf1e_simulator.effects import StatModifier, resolve_modifiers
+
+# ── Stacking types: sum together ──────────────────────────────────────────────
+
+
+class TestStackingTypes:
+    """Given: modifiers with stacking bonus types
+    When: resolve_modifiers is called
+    Then: values sum together."""
+
+    def test_two_dodge_bonus_stack(self) -> None:
+        mods = [
+            StatModifier("ac", 1, "dodge"),
+            StatModifier("ac", 2, "dodge"),
+        ]
+        assert resolve_modifiers(mods) == 3
+
+    def test_two_racial_bonus_stack(self) -> None:
+        mods = [
+            StatModifier("save", 1, "racial"),
+            StatModifier("save", 1, "racial"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+    def test_two_trait_bonus_stack(self) -> None:
+        mods = [
+            StatModifier("attack", 1, "trait"),
+            StatModifier("attack", 1, "trait"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+    def test_two_untyped_bonus_stack(self) -> None:
+        mods = [
+            StatModifier("attack", 1),
+            StatModifier("attack", 2),
+        ]
+        assert resolve_modifiers(mods) == 3
+
+    def test_mixed_stacking_types_all_sum(self) -> None:
+        mods = [
+            StatModifier("ac", 1, "dodge"),
+            StatModifier("ac", 2, "racial"),
+            StatModifier("ac", 1, "trait"),
+            StatModifier("ac", 1),
+        ]
+        assert resolve_modifiers(mods) == 5
+
+    def test_negative_untyped_stack(self) -> None:
+        mods = [
+            StatModifier("attack", -1),
+            StatModifier("attack", -2),
+        ]
+        assert resolve_modifiers(mods) == -3
+
+
+# ── Non-stacking types: keep highest ──────────────────────────────────────────
+
+
+class TestNonStackingTypes:
+    """Given: modifiers with non-stacking bonus types
+    When: resolve_modifiers is called
+    Then: only the highest value per type is kept."""
+
+    def test_two_morale_keep_highest(self) -> None:
+        mods = [
+            StatModifier("attack", 2, "morale"),
+            StatModifier("attack", 1, "morale"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+    def test_two_sacred_keep_highest(self) -> None:
+        mods = [
+            StatModifier("ac", 2, "sacred"),
+            StatModifier("ac", 3, "sacred"),
+        ]
+        assert resolve_modifiers(mods) == 3
+
+    def test_two_enhancement_keep_highest(self) -> None:
+        mods = [
+            StatModifier("ac", 2, "enhancement"),
+            StatModifier("ac", 4, "enhancement"),
+        ]
+        assert resolve_modifiers(mods) == 4
+
+    def test_different_non_stacking_types_each_apply(self) -> None:
+        mods = [
+            StatModifier("ac", 2, "morale"),
+            StatModifier("ac", 1, "sacred"),
+            StatModifier("ac", 3, "enhancement"),
+        ]
+        assert resolve_modifiers(mods) == 6
+
+    def test_circumstance_does_not_stack(self) -> None:
+        mods = [
+            StatModifier("attack", 2, "circumstance"),
+            StatModifier("attack", 1, "circumstance"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+
+# ── Penalties ─────────────────────────────────────────────────────────────────
+
+
+class TestPenalties:
+    """Given: penalties (negative values) with non-stacking types
+    When: resolve_modifiers is called
+    Then: least severe penalty is kept (max value)."""
+
+    def test_same_type_penalties_keep_least_severe(self) -> None:
+        mods = [
+            StatModifier("attack", -2, "morale"),
+            StatModifier("attack", -1, "morale"),
+        ]
+        assert resolve_modifiers(mods) == -1
+
+    def test_same_type_bonus_and_penalty_keep_most_beneficial(self) -> None:
+        mods = [
+            StatModifier("attack", 2, "morale"),
+            StatModifier("attack", -1, "morale"),
+        ]
+        assert resolve_modifiers(mods) == 2
+
+    def test_untyped_penalties_stack(self) -> None:
+        mods = [
+            StatModifier("attack", -1),
+            StatModifier("attack", -2),
+        ]
+        assert resolve_modifiers(mods) == -3
+
+
+# ── Mixed: stacking + non-stacking ────────────────────────────────────────────
+
+
+class TestMixedStacking:
+    """Given: a mix of stacking and non-stacking modifiers
+    When: resolve_modifiers is called
+    Then: stacking types sum, non-stacking types keep highest, totals combine."""
+
+    def test_dodge_plus_morale_plus_untyped(self) -> None:
+        mods = [
+            StatModifier("ac", 1, "dodge"),
+            StatModifier("ac", 2, "morale"),
+            StatModifier("ac", 1),
+        ]
+        assert resolve_modifiers(mods) == 4
+
+    def test_two_morale_plus_dodge_plus_racial(self) -> None:
+        mods = [
+            StatModifier("ac", 2, "morale"),
+            StatModifier("ac", 3, "morale"),
+            StatModifier("ac", 1, "dodge"),
+            StatModifier("ac", 1, "racial"),
+        ]
+        assert resolve_modifiers(mods) == 5  # 3 (max morale) + 1 (dodge) + 1 (racial)
+
+    def test_penalties_and_bonus_mixed(self) -> None:
+        mods = [
+            StatModifier("attack", 2, "morale"),
+            StatModifier("attack", -1, "morale"),
+            StatModifier("attack", -1),
+            StatModifier("attack", 1, "dodge"),
+        ]
+        assert resolve_modifiers(mods) == 2  # 2 (max morale) + -1 (untyped) + 1 (dodge)
+
+
+# ── Edge cases ────────────────────────────────────────────────────────────────
+
+
+class TestEdgeCases:
+    """Given: edge-case modifier lists
+    When: resolve_modifiers is called
+    Then: returns the correct total."""
+
+    def test_empty_list_returns_zero(self) -> None:
+        assert resolve_modifiers([]) == 0
+
+    def test_single_modifier(self) -> None:
+        assert resolve_modifiers([StatModifier("ac", 4, "dodge")]) == 4
+
+    def test_single_negative_modifier(self) -> None:
+        assert resolve_modifiers([StatModifier("attack", -3, "morale")]) == -3
+
+    def test_all_zero_values(self) -> None:
+        mods = [
+            StatModifier("ac", 0, "dodge"),
+            StatModifier("ac", 0, "morale"),
+        ]
+        assert resolve_modifiers(mods) == 0
+
+    def test_resolve_does_not_filter_by_target(self) -> None:
+        """resolve_modifiers treats all modifiers together regardless of target.
+
+        The caller must filter by target before calling — the function only
+        applies bonus-type stacking rules, not target grouping.
+        """
+        mods = [
+            StatModifier("attack", 2, "morale"),
+            StatModifier("ac", 3, "morale"),
+        ]
+        assert resolve_modifiers(mods) == 3  # same non-stacking type → max(2, 3)