Unlike most types of actions, a PC is allowed to use a reaction on another player’s turn or a monster’s turn. However, there aren’t many abilities that use a reaction, so most players won’t be able to take advantage of their reaction all that often, especially at low levels.
Opportunity Attacks
A common example of a reaction that any PC can take is an Opportunity Attack. An Opportunity Attack is a single attack made with a melee weapon, which you can only take if an enemy moves out of your Reach when you are wielding a melee weapon. This usually occurs because an enemy moves away from you, or because the try to move past you. Note: most melee weapons have a reach of 5 or 10 feet. Similarly, enemies are allowed to take an Opportunity Attack if a PC leave their range. Since most PCs and monsters only have one reaction, they can usually only take one opportunity attack per round. It’s also possible to avoid taking an opportunity attack by using the Disengage action, or through other special abilities.
Other Abilities Using a Reaction
Unlike Opportunity Attacks, most abilities that use a Reaction are granted by class or subclass features, spells, or feats. There are only a couple of Reactions that all players can use. These include: Using a reaction to trigger the Ready action, which is an action that a PC has prepared in advance to use when a specific triggering event occurs, like shooting a bow when an enemy comes within range. If you are riding a mount and if that mount is knocked prone, you can use a reaction to dismount and land on your feet. Or you can identify a spell as it’s being cast (optional rule from Xanathar’s Guide to Everything) with an Intelligence (Arcana) check of DC 15 + spell level.
What is Action Economy?
Action Economy is a term to indicate how effective a player is at accomplishing multiple things during each round. Skilled players will often try to find ways to use both their reaction and bonus action, which can make their characters more effective and more deadly in combat. It’s not always easy to do this, but there are also a number of class/subclass features, feats, and spells that players can use to improve their action economy. For example, a popular feat is Polearm Master, which allows a PC to (a) make a bonus action attack on their turn, and (b) take an Opportunity Attack when an enemy moves into their range (which is 10 feet for many of the weapons mentioned in the feat). As you can see, this feat is great for action economy because it gives both a bonus action and a reaction option, which can let players make up to 3-times more attacks than they normally would. However, there are many other ways to use reactions, especially as characters gain levels. Certain spells use a reaction, like shield, absorb elements, hellish rebuke, feather fall, and counterspell. In each of those cases, a specific triggering event needs to occur. For example, an enemy needs to attack on you in order to cast Shield, whereas an enemy needs to damage you in order to cast Hellish Rebuke. Rogues are masters of Action Economy, because their Cunning Action gives them several bonus actions abilities, and Uncanny Dodge lets them use a reaction to take half-damage from one attack each turn, provided the attack is visible to the rogue.
Rounds and Turns in 5e
In 5e, every character has one action and one bonus action they can use on their Turn, and one reaction that they can use during each Round of combat. A Round is made of a series of Turns. Each player gets their own Turn during which they can move and take an action, and a bonus action (if any). Enemies also get one or more Turns where they can move and act. The order of Turns is usually determined by rolling for Initiative. Since a Round is made of many Turns, a player can use a reaction on another character’s turn or during an enemy’s Turn if there’s a triggering effect. A player can also use a reaction on their Turn, provided there’s a triggering effect. For example: if a player moves and takes an Opportunity Attack, they can use their reaction to cast the Shield spell. However, they’re not allowed to do this if they’ve already cast a Bonus Action spell, because the rules for casting a spell using a bonus action state that “you can’t cast another spell during the same turn, except for a cantrip with a Casting Time of 1 action.” On the other hand, a wizard can cast a spell using a regular action and a reaction during the same turn (weird but true.)
Some Basic Reaction Rules
If you have not used your reaction before your turn starts, you lose the previous reaction and start anew. So your reaction resets at the beginning of your turn. In addition, PCs and enemies are not allowed to use a Reaction if they’re Surprised at the beginning of combat until the end of their first Turn. They’re also not allowed to move or to take an action until the end of their first Turn.
The Bottom Line
Don’t worry if you can’t find a way to use your Reaction all that often! However, you should also be aware of any bonus actions that you have, and look for ways to use them, especially as you gain levels. Some of the newer subclasses (such as those in Xanathar’s Guide to Everything or Tasha’s Cauldron of Everything) are slightly more likely to have reaction abilities than the subclasses in the Player’s Handbook. Spellcasters should seriously consider taking at least one reaction spell if they have a decent choice. However, casting a spell using a reaction still uses up a spell slot, so you probably don’t want to overuse reaction and bonus action spells.
