Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higherlevel spells are even more so. Thus, each spellcasting class's description (except that of the warlock) includes a table showing how many spell slots of each spell level a character can use at each character level. For example, the 3rd-level wizard Umara has four 1st-level spell slots and two 2nd-level slots.

When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively "filling" a slot with the spell. You can think of a spell slot as a groove of a certain size — small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size, but a 9th-level spell fits only in a 9th-level slot. So when Umara casts Magic Missile, a 1st-level spell, she spends one of her four 1st-level slots and has three remaining.

Finishing a long rest restores any expended spell slots.

Some characters and monsters have special abilities that let them cast spells without using spell slots. For example, a monk who follows the Way of the Four Elements, a warlock who chooses certain eldritch invocations, and a pit fiend from the Nine Hells can all cast spells in such a way.

Casting a Spell at a Higher Level

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts Magic Missile using one of her 2nd-level slots, that Magic Missile is 2nd level. Effectively, the spell expands to fill the slot it is put into.

Some spells, such as Magic Missile and Cure Wounds, have more powerful effects when cast at a higher level, as detailed in a spell's description.

Casting in Armor

Because of the mental focus and precise gestures required for spellcasting, you must be proficient with the armor you are wearing to cast a spell. You are otherwise too distracted and physically hampered by your armor for spellcasting.