Attacks

For a comprehensive summary of the mechanics behind all rolls and modifiers, see Die Rolls.

An Attack Roll happens when the game wants to decide whether an attack will hit its target. The Attack Roll uses a D20 and can be affected by Modifiers such as the Ability Score Modifier and Proficiency Bonus of the creature making the attack.

Result =  d20 + Ability Score Modifier + (optional) Proficiency Bonus

The result of the roll is compared to the Armor Class of the target. If the Armor Class is reached or exceeded, the attack is a successful hit, and the game proceeds with a Damage Roll to determine the amount of damage dealt.

Attacks done with Weapons always use an Attack Roll to determine success. Some spells also use an Attack Roll to determine whether they hit their intended target (such as Fire Bolt or Eldritch Blast), though other spells hit the target automatically (such as Magic Missile or Fireball). A spell that hits its target automatically may allow the target to make a Saving Throw instead.

Critical Miss / Critical Hit

As a special case, a roll of 1 means that the attack fails regardless of bonuses and target Armor Class. This is called a Critical Miss. Likewise, a roll of 20 means the attack succeeds regardless of the target's Armor Class. This is called a Critical Hit. A critical hit also affects the result of the Damage Roll, though the details of how this will be implemented in the finished game are not known yet.

Modifiers

Attack Rolls can be affected by Advantage / Disadvantage, by the attacker's Ability Score Modifier, and the attacker's Proficiency Bonus.

Ability Score Modifier

A bonus or penalty may be applied to the result of the roll based on an Ability Score. Which ability is used depends on the type of attack being made.

Melee and ranged weapon attacks use Strength or Dexterity, depending on the weapon. Usually, Strength for melee weapons, and Dexterity for ranged weapons. The exceptions are Finesse weapons, where the higher one of Strength and Dexterity will be used; and Thrown weapons, which use Strength for both melee and ranged attacks. If a weapon is both Thrown and a Finesse weapon, it will use the higher of Strength and Dexterity both for melee and ranged attacks.

For spells, the Ability Score used for the Modifier depends on the class of the caster:

Table of classes by which Ability Score they use for spells

Intelligence	Wisdom	Charisma
Wizard Arcane Trickster (Rogue) Eldritch Knight (Fighter)	Cleric Druid Ranger	Bard Paladin Sorcerer Warlock

The bonus or penalty that will be applied is based on this table:

Ability Score Modifier depending on the value of the used Ability Score

Score	Modifier
1	-5
2-3	-4
4-5	-3
6-7	-2
8-9	-1
10-11	+0
12-13	+1
14-15	+2
16-17	+3
18-19	+4
20-21	+5
22-23	+6
24-25	+7
26-27	+8
28-29	+9
30	+10

Proficiency Bonus

The attacking creature's Proficiency Bonus is added to the result of the roll if the creature has the correct kind of Proficiency for the attack it's making.

For weapon attacks, the creature must have proficiency in the weapon it's using to make the attack.

For spell attacks, the creature must have innate spell casting capabilities granted by its Class, Subclass, or Feats.

The lack of spell casting proficiency is rarely noticed, since any creature innately capable of casting spells has it. The lack of this proficiency only becomes apparent when items, such as Scrolls, are used to cast spells by a creature which otherwise couldn't cast any spells.