Part of an ongoing series of 5e (2014) Rules notes.  Notes on 5.5e (2024) rules are included below.

Questions about this pop up sooner or later in every campaign, as it’s an area that 5e either does fine or poorly, depending on which online forum you go to or  specific question you examine.

So what happens when instead of bashing each other with hunks of steel or eldritch energies, you get up-close and personal

You can attack to Grapple

This is discussed on PHB 195. Essentially you use one of your Attacks to make a skill contest:

• Attacker: roll Strength (Athletics)
• Defender: roll Strength (Athletics) or Dexterity (Acrobatics)

In other words, Grapplers need to be a lot better at that skill, or be pretty darned strong.

If you win, the target is Grappled (technically, has the Grappled condition, PHB 290).

Which, honestly, doesn’t do much.  It basically reduces Speed to 0, even if the target has other features that increase its speed beyond base.

But a grappled target can still punch grappler, stab them, etc. They are more restrained by being within 5′ of their grappler (which gives them Disadvantage on spell attacks or ranged attacks), but melee attacks are just fine.

Grappling does keep the grappled target from running away until help arrives or until the target breaks the Grapple. They can try that by  taking their Action to roll Strength (Athletics) or Dexterity (Acrobatics) vs the grappler’s Strength (Athletics) again. 

The grappler can also slowly (half speed) drag a grappled target with them — or continue attacking them at close range (with no particular advantage, and with the same disadvantages).

Note:

• When you have Grappled someone, you are not yourself considered Grappled.
• When you have Grappled someone, you are using up one hand (the other hand is considered free for attacks, etc.)

Aside from breaking free of the Grapple through a contest, it can also be broken by:

• The grappler being Incapacitated.
• An effect knocking the grappled target out of the reach of the grappler or the Grappling effect, e.g., being knocked away by a Thunderwave.

Note there is a Grappler feat you can take. This gives you Advantage on attack rolls against someone you are grappling, and the opportunity to pin (restrain) someone (through another grapple check).

5.5e handles this a little more simply.

Grappling is part of the Unarmed Strike category, but instead of doing damage on a hit, and rather than an Athletics/Acrobatics contest (5.5e hates contests), the target has to make a Strength or Dexterity save vs a DC of (8 + attacker’s STR mod + attacker’s Proficiency Bonus). It’s simpler dice rolling, though the attacker will need to provide some numbers to make it work.

Once grappled, a player can keep the grapple, attack the target (with a hand not in use), move the target, or release.

The target can escape by using their action for a Strength (Athletics) or Dexterity (Acrobatics) check vs. that same DC.

The Grappled condition in 5.5e adds to it:

• Grappled creatures have Disadvantage to attack rolls against anyone other than their grappler.
• If you move someone you have grappled, instead of halving your speed, each foot costs an extra foot of speed — similar to difficult terrain, which means it stacks with it. (This doesn’t apply when the Grappler is two sizes or more larger than the Grappled.)

Once Grappled, someone can use a Utilize and a pair of manacles to bind a Small/Medium Grappled (or Incapacitated, or Restrained) creature on a Dexterity (Sleight of Hand) check against DC 13.

The Grappler feat is also rewritten. Once per turn if you hit with an Unarmed Strike, you can both do damage and try to Grapple them. You have Advantage to attack creatures you have grappled, and you can move them around at normal speed (if your size or smaller). (It also gives you +1 ASI for Strength or Dexterity.)

What if more than one person at a time is Grappling you?

While executing a Grapple takes one Attack (of however many you have within your Attack action), escaping a Grapple takes an entire Action. If more than one person has you Grappled, you can only escape one Grapple per turn … which means tag-teaming Grapplers can seriously cause you a problem.

Monster and Spell Grapples and Restraints

This is where things get a little tricky. Or more straightforward. You decide. The Grappled and Restrained conditions are quite separate, but come up a lot in spell and monster attacks: e.g., a Giant Octopus you Grappled with its tentacles; a Web spell causes the Restrained condition (PHB 292).

• Just like with a Grapple, a Restrained creature’s speed becomes 0, and it can’t use any bonus to its speed.
• Attack rolls against the Restrained  target have Advantage, and the their attack rolls have Disadvantage.
• The Restrained target also has Disadvantage on Dexterity saving throws.

(5.5e does the same thing with its Restrained condition.)

(Note that the Grappler feat lets the grappler get Advantage on the Grappled target — but, more interestingly, it lets a grappler roll a second Grapple attack, which, if it succeeds, means both the grappler and the grappled target are both Restrained. Which actually seems kinda sucky.)

So, Dave, how do I do something useful with this?

Grappling by itself is kind of limited in what it can do. But with a little work, it can make an effective attack.  So  how do you grab an opponent and actually subdue him.

The vast consensus I’ve read on this is Grapple them (so they can’t get away) and then do a Shove attack (PHB 195, same Ability checks as the Grapple) to knock them Prone …

…  at which point they get the Prone condition’s effects (PHB 292), and as Prone:

• they can only crawl (but not if they’re Grappled and at speed 0!)
• or they can get up (but not if they’re Grappled and at speed 0!)
• they are at a Disadvantage to attack (Prone!)
• attacks on them have Advantage (Prone!)
• … and they probably have to use their whole Action to try to desperately break the Grapple.

People have created whole builds around this.

Are you, the attacker, also Prone at that point? A good question. Consensus seems to be “No, but describe what you are doing to your DM.” I’d think of it as the arm twist behind the guy on the ground; you’re enough on your feet that you are not considered Prone, and can defend attacks normally (albeit with one hand), etc.

Given that, as long as you maintain this, you can continue attacking the Grappled+Prone guy and other folk can attack them, too, with Advantage, and that should distract them from attacking you back (as they desperately try to break the Grapple).

Consensus is that Grappling is much better (when by the players) in 5.5e (2024) than 5e. Monks, especially, are seen as natural beneficiaries of this.