Part of an ongoing series of 5e Rules notes.

It’s not always possible (or best idea) to just attack-attack-attack. Sometimes you want to … take some time, to seize the right moment, to make sure you understand the situation and can best act on it.

That’s what the Ready Action is for, probably one of the most confusing actions for new (or new, veteran) players.

So, what is the Ready action?

Part of the confusion here is that previous D&D versions have had the concept of “holding” or (4e) “delaying” a turn (“Go ahead and skip over me on Initiative for the round, I’m going to just hold on until I more clearly see what is happening and then act accordingly”). 5e reframes and reduces that concept to Readying an Action. It’s much more constrained as (a) it requires defining a trigger-response pairing, and (b) if the trigger doesn’t happen, your Action for that turn is lost.

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready Action on your turn, which lets you act using your Reaction before the start of your next turn.

So an important couple of things here. First you are waiting for a particular circumstance, and second, you are not only using your turn’s action for Ready, but it will depend on you having your Reaction left in order to use it. And if the circumstance doesn’t occur … you’ve essentially “wasted” your actions for the turn. (Yes, “they also serve who stand and wait,” but it’s still kind of disappointing.)

A different way of looking at a Readied Action is that it’s a called Reaction to a specific circumstance (vs a canned Reaction such as an Opportunity Attack or a spell that can be cast as a Reaction).  You don’t so much declare an Action as declare a Reaction you are going to take before your next turn. (I wonder if it would be a bit less confusing if they called it “Ready Reaction” rather than “Ready Action”.)

First, you decide what perceivable circumstance will trigger your Reaction. Then, you choose the Action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include “If the cultist steps on the trapdoor, I’ll pull the lever that opens it,” and “If the goblin steps to me, I move away.”

So the trigger has to be a perceivable circumstance. You can’t Ready an Action  in response to something you are unaware of.

(Can you make some sort of Perception / Investigation / Insight check as part of that? Good question. The answer is no as an Active check, because that’s an Action (the Search Action, to be specific); the DM could let you do a Passive check, because that’s automatic. But, then, that whole Active/Passive check thing gets complicated, especially when combined with Hidden stuff.)

The Reaction to that perceivable circumstance is then to either take a single Action (if you are a high enough level fighter to have multiple Attack Actions, you can still take only one), or you can move.

(Note: don’t wait until the goblin steps up to you to move, or else they will get an Opportunity Attack on you; wait until the step within ten feet of you.)

When the trigger occurs, you can either take your Reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one Reaction per round.

A Readied Action is binary: when the trigger occurs you must decide to take that Reaction right then, or decide to ignore it and the Readied Action goes away (though it doesn’t count as having lost your Reaction for purposes of other types of Reactions).

If you say, “If a goblin steps out of cover over there, I will throw my javelin,” then once the goblin charges out of cover, you can’t wait until it gets to as closer range to shoot: you have to take the shot right then and there.

When you Ready a Spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs.

To be Readied, a spell must have a casting time of 1 action, and holding onto the spell’s magic requires Concentration. If your Concentration is broken, the spell dissipates without taking effect.

For example, if you are Concentrating on the Web spell and Ready Magic Missile, your Web spell ends, and if you take damage before you release Magic Missile with your reaction, your concentration might be broken.

This is also important; a Readied spell burns the spell, whether or not the Reaction is triggered or you choose to let it be triggered.  For this reason, a lot of magic-users only Ready cantrips, since nothing is “lost” if it isn’t used.

So, in summary:

• Attacks are limited to a single attack, as with Opportunity Attacks. Even if you normally get three attacks on your turn, you can only Ready one attack. The Extra Attack ability notes that it only applies on your turn. Read more about Actions and Attacks here.
• It only allows Movement or an Action (not a Bonus Action, as Bonus Actions only occur on your turn).
• That said, you can Move and take a Bonus Action and still take a Ready action, all on your turn. So it doesn’t mean you are completely paralyzed.
• You only get one Reaction between turns. If you make an Opportunity Attack, or any other Reaction (like Counterspell) before your Readied Action triggers, you lose your Readied Action (and vice-versa). On the other hand, if you managed to use your Readied Action, you cannot take another Reaction.
• Readied spells actually burn the spell slot (if any) upon Readying, and require Concentration to hold onto until the trigger occurs (if it ever does), interrupting any other Concentrated-upon spells and possibly being lost if you fail a required Concentration check before it’s triggered.

What conditions can you Ready for?

To my mind, the “perceivable circumstances” for the Readied action require a something you can focus on — a place, a person, a proximity.

Examples that seem to me to be legit Readied (Re)actions

(beyond the ones mentioned in the actual Ready text above):

• If an opponent steps into the doorway, I will Flamebolt them. [place]
• When that archer steps back out from behind the tree to shoot me, I will shoot them. [place]
• If an opponent steps next to me, I will hit them. [proximity]
• Once the paladin engages an opponent, I will shoot their opponent. [person] [Rogues, this could allow a Sneak Attack, as that is allowed on any turn, not just yours.]
• If an opponent steps onto the trap door, I will pull the lever. [place]
• If an opponent gets within ten feet of me, I will run toward the door. [proximity]
• Whichever of the two people I am standing next to first has someone step next them I will attack their attacker them. [person]
• If Bob gets out of that cluster of bad guys, I’ll drop a Fireball there. [place]
• If the guard pulls out her sword, I’ll stab her. [person]

Examples that seem to be to be too broad or complex or rule-bending to be an Readied action:

• If an opponent steps next to me then I will Disengage and move toward the door. [You can act or you can move. Disengage is an action, and does not include actual movement]
• If an Orc comes through the door, I will Flamebolt them. But if it’s an Orcish captain, I’ll Fireball them. [You can only prep one spell, and discernment as to Orcish rank is probably more complex than than you can take as a Reaction.]
• When the Orcs arrive, If I am attacked I will Dodge, but if not, then I will Help my neighbor [You can’t plan more than one action or circumstance. Just Dodge instead.]
• If an opponent gets within ten feet of me, I will Dash away. [Movement from a Ready is only up to your normal full movement, and is an alternative to taking an Action; Dash would be an Action, but it doesn’t actually move you, it just changes how far you can move.]
• If people come out of one of the other three doors to the room, I shoot them. [Unless the doors are right next to each other, that’s too much to keep track of for a trigger; I tend to rule that focus of that sort is, most, over a 90 degree arc, about how broadly you can really see without moving your head.]

Remember that you are not obliged to follow through on a Readied action; you have enough Reaction time to either do the Readied action or to abort it (which, for  a spell, means the slot has still been used up).

Spells

Also note that, for Readying a spell, that takes Concentration to maintain the readiness (you basically casting but holding the spell), so any Concentration spell you have running would drop when you do so. This is true:

1. Even if the spell you are casting doesn’t normally require Concentration — it’s the Readying and holding of it that requires the Concentration.
2. Even if you don’t eventually fire the spell — it’s the Readying that burns the spell and starts the Concentration.

There is an interesting extrapolation that since a Readied spell is actually cast on the player’s turn, not on the Ready-triggered release toward an enemy, then Counterspell would need to be cast as a Reaction on the casting, not the release. Which is really weird as it implies that the attacker knows you’ve cast (say) Fireball before it actually goes off, and is especially pernicious with an attack of Readying / casting a spell outside of Counterspell range, then running close enough to be able to actually release it as the trigger condition. Counterspell already makes my head hurt with its causality issues, so I’m going to try not to think about it.

So why does Ready work this way?

The Sage Advice Compendium goes into a bit of detail as to why things were designed this way, vs. previous editions (esp. 4e) that allowed folk to simply delay their position on the Initiative list in a round. It doesn’t change gameplay, but it’s still kind of interesting to understand the design goals.

For a variety of reasons, we didn’t include the option to delay your turn:

• Your turn involves several decisions, including where to move and what action to take. If you could delay your turn, your decision-making would possibly become slower, since you would have to consider whether you wanted to take your turn at all. Multiply that extra analysis by the number of characters and monsters in a combat, and you have the potential for many slowdowns in play.
• The ability to delay your turn can make initiative meaningless, as characters and monsters bounce around in the initiative order. If combatants can change their place in the initiative order at will, why use initiative at all? On top of that, changing initiative can easily turn into an unwelcome chore, especially for the DM, who might have to change the initiative list over and over during a fight.
• Being able to delay your turn can let you wreak havoc on the durations of spells and other effects, particularly any of them that last until your next turn. Simply by changing when your turn happens, you could change the length of certain spells. The way to guard against such abuse would be to create a set of additional rules that would limit your ability to change durations. The net effect? More complexity would be added to the game, and with more complexity, there is greater potential for slower play.

Two of our goals for combat were for it to be speedy and for initiative to matter. We didn’t want to start every combat by rolling initiative and then undermine turn order with a delay option. Moreover, we felt that toying with initiative wasn’t where the focus should be in battle. Instead, the dramatic actions of the combatants should be the focus, with turns that happen as quickly as possible

In short, the 5e designers decided that somewhat more elaborate, and limited, rules for Readying Actions would actually make the game flow smoother, quicker, and in a less complicated fashion. I can’t say that I disagree.