Aerial Combat

Many monsters can fly, and PCs can use spells and items to gain the ability to fly. Flying creatures have to use the Fly action to move through the air. Performing an especially tricky maneuver might require using Acrobatics to Maneuver in Flight. Creatures might fall from the sky, using the falling rules. At the GM's discretion, some groundbased actions might not work in the air. For instance, a flying creature couldn't Leap.

In aerial and aquatic combat, you might need to track positioning in three dimensions. For flying creatures, you might use one of the following methods.

Find platforms to place flying creatures' miniatures on.
Set a die next to a creature with the number indicating how many squares up in the air it is.
Make a stack of dice or tokens, 1 per 5 feet of elevation.
Write the elevation next to the monster on the grid.

In underwater combat, choose a plane to be the baseline, typically the waterline, the sea floor, or a stationary object you can measure from.
As with ground-based movement, moving diagonally up or down in 3-D space requires counting every other diagonal as 10 feet. Measure flanking in all directions— creatures above and below an enemy can flank it just as effectively as they can from opposite sides.