When a player who has advantage on a d20-based roll, they roll twice and take the highest result. When they have disadvantage, they roll twice and take the lowest result. Multiple instances of advantage or disadvantage do not stack, and if a player has both advantage and disadvantage on the same roll, they cancel out.
Prior to Dungeons & Dragons 5th edition, players making d20-based rolls such attacks, saving throws, or skill checks, sometimes applied a numeric bonus or penalty to the roll in order to represent some circumstance which would favor or hinder them. Examples might include attacking an opponent who is obscured by poor lighting, or sneaking past someone who is distracted.
Large numbers of small bonuses sometimes became onerous to track, particularly in D&D 4th edition, where many powers applied some small bonus or penalty. These also created a circumstance where someone who accumulated several small bonuses could achieve a much higher result than normal.
Advantage and disadvantage appeared early in the playtest of D&D Next, the rules system which would later be known as Dungeons & Dragons 5th edition. They appeared in the first public playtest rules for the edition, released on May 24, 2012. Advantage/disadvantage were inspired by action points, a mechanic often used to reroll a missed attack.
In 2013, Mike Mearls described the mechanic's origin:
- During our very earliest tests of D&D Next, the advantage mechanic grew out of this imperative. In past editions, we've used tables big and small to capture all the +1 or –2 modifiers that can creep into the game. Advantage (along with its sinister twin, disadvantage) is easy to remember, simple to apply before or after a roll, and comprehensive enough to devour huge swaths of fiddly modifiers.
- We went through a lot of arguments and ideas over how to implement a variety of penalties and bonuses in the game—along with calls to simply do away with the entire concept or approach it from a much more radical angle. In looking at the static that modifiers caused, it was clear that removing them from the game eliminated many issues. The game was faster, there were fewer exceptions to memorize, the DM had fewer things to track, and the game became much simpler to explain to beginners or players who didn't care much for memorizing complex rules.
- This case also shows how one rule can indirectly lead to another. The action points of 3rd Edition and 4th Edition inspired the concept of advantage. Using an action point allows you to take an additional action. Most often, you might make an attack, miss, and then spend an action point to make that same attack again. Action points essentially allow a small-scale do-over, and that concept of a reroll as a bonus morphed into D&D Next's advantage mechanic.