Spellcasting Rules: Complete Reference
Spellcasting sits at the mechanical heart of Dungeons & Dragons 5th Edition — governing everything from a wizard's Fireball to a paladin's Divine Smite. The rules span multiple interlocking systems: spell slots, components, concentration, saving throws, and attack rolls. Getting them right matters because spellcasting interactions come up in roughly every other combat round at most tables, and a single misread rule can quietly reshape the power balance of an entire campaign.
Definition and scope
A spell in D&D 5e is a discrete magical effect defined by four fixed properties: a school of magic (one of 8 schools, including Evocation, Illusion, and Necromancy), a level from 0 (cantrip) to 9, a casting time, and a range. The rules governing how spells function are codified in Chapter 10 of the Player's Handbook (Wizards of the Coast, 2014), which remains the primary reference document for spellcasting mechanics.
Spellcasting scope covers every character who casts spells — full casters like wizards and clerics, half-casters like paladins and rangers, and hybrid casters like the eldritch knight. Each class accesses spells differently. Wizards prepare from a spellbook, sorcerers know a fixed list, and clerics prepare from their entire class list each day. That structural difference between "known" spells and "prepared" spells trips up more new players than almost any other rule.
For a broader sense of how magic fits into the game's overall design, the key dimensions and scopes of D&D page provides useful framing.
How it works
Every spell requires three things before it resolves: the caster must have the spell available, must expend the appropriate resource, and must satisfy the spell's components.
Spell slots are the primary resource. A 5th-level wizard has 4 first-level slots, 3 second-level slots, and 2 third-level slots per long rest (per the Player's Handbook spell slot table). Slots are expended when a spell is cast, not when it's prepared. A prepared spell can be cast using any slot of sufficient level — a Magic Missile cast in a 3rd-level slot produces 5 darts instead of the base 3.
Components break into three types:
- Verbal (V) — the caster must be able to speak; silence effects block this
- Somatic (S) — the caster must have a free hand; grappling or restraints can matter here
- Material (M) — a physical component, sometimes with a verified gold cost; a spellcasting focus can substitute for components without a cost
Once cast, a spell either resolves instantly or requires concentration. Concentration spells — Hold Person, Fly, Hypnotic Pattern — last up to their verified duration but end immediately if the caster loses concentration. Damage forces a Constitution saving throw (DC 10 or half the damage taken, whichever is higher) to maintain concentration. Only 1 concentration spell can be active at a time, full stop.
Spell attack rolls apply when a spell requires the caster to hit a target: roll a d20, add the spell attack modifier (proficiency bonus + spellcasting ability modifier), and compare to the target's Armor Class. Saving throw spells skip the attack roll — the target rolls instead, and failure triggers the effect. The caster's spell save DC is 8 + proficiency bonus + spellcasting ability modifier.
Common scenarios
Counterspell interactions generate more table debate than almost any other rule. A reaction cast in response to a creature casting a spell of 3rd level or lower automatically succeeds; against 4th level or higher spells, the counterspell caster makes an ability check using their spellcasting ability (DC 10 + spell level). Notably, counterspell targets the casting, not the caster — it can be countered itself.
Area of effect targeting requires attention to whether a spell targets a point of origin (like Fireball's 20-foot radius sphere from a point within 150 feet) or a line, cone, or cube. Cover and line-of-sight rules interact with spell targeting: a creature behind total cover can't be directly targeted by a spell, but area spells that spread around corners can still catch them.
Upcasting — casting a spell using a higher-level slot than the minimum — enhances a spell only if its description includes an "At Higher Levels" entry. Cure Wounds benefits from upcasting; Detect Magic does not.
The D&D frequently asked questions page addresses several of the most common edge cases that don't resolve cleanly from the rulebook text alone.
Decision boundaries
The trickiest judgment calls in spellcasting fall into a consistent pattern: timing, stacking, and interaction with conditions.
Timing matters for reactions. Shield (a reaction) can be cast in response to being hit — but the hit must be declared before the shield resolves. The Dungeon Master announces the hit, the player declares Shield, and the AC retroactively rises; if the new AC exceeds the attack roll, the hit misses.
Stacking has a clear rule: two instances of the same spell don't stack, but different spells providing the same bonus type may or may not depending on whether they specify a bonus type (circumstance, enhancement, etc.). In 5e, two Bless spells on the same creature produce no added benefit — only one instance applies.
Concentration vs. instantaneous is the line that separates the most powerful spells from everything else. Wish is instantaneous. Simulacrum is not. Fireball resolves immediately; Wall of Fire demands sustained concentration. This distinction is the primary design lever Wizards of the Coast uses to price spell power.
The how it works overview page covers how these mechanical layers connect to the game's broader action economy. For players building their first spellcasting character, the how to get help for D&D page points toward community resources where ambiguous rulings get hashed out by experienced players and Dungeon Masters.