DnD Combat Rules: A Complete Reference
Dungeons & Dragons combat is a structured, turn-based system that transforms chaotic fictional violence into a precise sequence of decisions, dice rolls, and consequences. This page covers the full mechanics of the combat framework as written in the 5th Edition rules — from initiative to the final blow — along with the judgment calls that arise when players push against the edges of what the rulebook anticipated. Whether a session involves a tavern brawl or a boss fight with a CR 24 ancient dragon, the same underlying engine drives it.
Definition and Scope
Combat in D&D 5th Edition is defined as any situation where the rules shift from open exploration into the structured initiative order — a formalized turn sequence that governs who acts when. This triggers the moment a hostile action is declared or a creature is surprised, and it ends when all combatants on one side are dead, incapacitated, fled, or have surrendered.
The combat system is documented primarily in Chapter 9 of the Player's Handbook (Wizards of the Coast, 2014), which establishes the three-round structure of each turn: Bonus Action, Action, and Movement. A character also has a Reaction, which can fire once per round outside the normal turn order — the most common use being the Attack of Opportunity.
The scope of the system is deliberately broad. It handles melee brawls, ranged engagements, mounted combat, underwater fights, and spell-dominated encounters using the same core framework. That flexibility is intentional, and it's also the source of nearly every rules dispute a table will ever have. For broader context on how combat fits into the game's structure, see Key Dimensions and Scopes of DnD.
How It Works
Combat begins with each participant rolling initiative — a d20 plus their Dexterity modifier. This determines turn order for the entire encounter. Ties are broken by comparing Dexterity scores, and if those are equal, the tied parties roll off.
On each turn, a creature can do the following:
- Move up to its full speed (typically 30 feet for humans and most humanoid races)
- Take one Action — which can include attacking, casting a spell, using a skill, or taking the Dash, Disengage, or Dodge action
- Use one Bonus Action, if a class feature, spell, or ability grants one
- Use one Reaction, triggered by a specific condition (such as an enemy leaving melee range)
The central mechanic of most actions is the attack roll: roll a d20, add the relevant modifier (Strength for melee, Dexterity for ranged or finesse weapons, Spellcasting Ability for spells), and compare the result to the target's Armor Class (AC). A result equal to or higher than the AC hits. On a natural 20, the attack is a critical hit, and all damage dice are rolled twice.
Damage is subtracted from Hit Points. At 0 HP, a creature falls unconscious and must begin rolling death saving throws — a DC 10 Constitution save repeated each turn. Three successes stabilize the character; three failures mean death. A critical hit from an enemy while downed counts as two failures at once, which is the kind of rule that tends to get forgotten at the worst possible moment.
The how-it-works page on this site explores the mechanical engine underlying the broader game, including the d20 probability curve that makes a +1 bonus meaningfully different from a +2 at the margins.
Common Scenarios
Grappling operates as a special attack action: the attacker makes an Athletics check contested by the target's Athletics or Acrobatics. On a success, the target's speed drops to 0. This is a contested ability check, not an attack roll — which matters for features that trigger on "attacks" specifically.
Flanking is an optional rule from the Dungeon Master's Guide (p. 251), not a core rule. Tables that use it grant advantage on attack rolls when two allies are on opposite sides of a target. Tables that don't often don't realize it's optional, which accounts for a significant share of DnD frequently asked questions.
Concentration applies to a large portion of 5th Edition spells. Casting a second concentration spell ends the first automatically. Taking damage while concentrating requires a Constitution saving throw with a DC of 10 or half the damage taken, whichever is higher.
Area-of-Effect Targeting — spells like Fireball affect a 20-foot radius sphere. Creatures who succeed on a Dexterity saving throw (DC set by the caster's spell save DC) take half damage. This is one of the cleaner interactions in the system, though it breaks down slightly when tight formations and allied positioning create collateral fire questions.
Decision Boundaries
The rules draw a clean line between Actions and Bonus Actions: they are not interchangeable. A spell that requires a Bonus Action to cast restricts the same turn to cantrips only for any additional spell casting (PHB, p. 202). This constraint stops certain action-economy builds from accelerating far beyond what the encounter math assumes.
A harder boundary is concentration versus non-concentration spells. Spells like Bless (concentration) and Magic Missile (non-concentration) operate under completely different constraints even though they're both 1st-level spells, and treating them identically is one of the most common mid-level table errors.
The trickiest judgment calls involve readied actions — when a player holds their action until a trigger occurs. A readied action uses a Reaction, not an extra turn. A readied spell requires maintaining concentration until the trigger fires. If the trigger never comes, the spell slot is expended regardless. The DnD frequently asked questions page addresses several edge cases that fall into this category.
For players navigating these rules for the first time, or DMs trying to adjudicate contested situations at speed, how to get help for DnD outlines the most reliable reference communities and official errata channels.