Concentration is a core part of all spellcasting; some of the most impressive acts of magic are only obtained through intense focus. Though not all spells require concentration–stronger spells with a duration longer than a single turn generally do, balancing the power of the spell with the chance to end it early by breaking concentration and preventing it being used with similar spells.
How Concentration works
When you cast a spell which requires concentration, you’re focusing intensely to maintain that spell. Though not all spells require concentration, a spell will have concentration listed next to the duration, if it requires it. Said spells can be held for as long as you like, up to that listed duration. Your concentration can end under the following circumstances:
You can wilfully drop your concentration at any time (no action required),
If you’re incapcitated or killed
If you cast another concentration spell
Through damage. When you take damage, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage taken, whichever is higher, you must make this saving throw every time you take damage, e.g. if a warrior hits you twice with their sword, you’d have to make two saving throws.
Your DM may choose to ask for a saving throw to maintain concentration based on something in the environment, such as a wave washing over you on a ship; these saves will require a DC 10 to pass.
You can’t concentrate on more than one spell at a time, so be aware of your concentration. Your concentration can also be occupied by certain abilities, such as the College of Glamour Bard’s Mantle of Majesty ability, and by readying an action to cast a spell. When you ready a spell you cast it on your turn and hold it until your next turn or the trigger condition you decide is met, whilst you are holding that spell you are concentrating on it even if that spell does not normally require concentration. There’s also edge cases like the Barbarian’s Rage feature, which prevents you from maintaining concentration whilst using it.
Maintaining spells can be a nuisance, especially when up against an enemy who really likes Magic Missile spell, or has a lot of attacks (or worse, the Mage SlayerFeat), as each instance of damage means one more saving throw in order to not have your spell fizzle out. Worry not, however, here’s a couple of ways you can counter this problem: increase your Saving Throw Bonus for Constitution; or by take the Warcaster Feat which, among other things, grants Advantage on Saving Throws to maintain concentration.
Note! If a magic item or feature allows you to cast a concentration spell, that doesn’t mean you get a freebie–you still need to concentrate on it.
That’s just about everything you need to know about concentration in 5E, let’s hope you concentrated whilst reading it. If you want more knowledge about DnD, dive deeper into Dive Cove!