Boos are ghost-like creatures that appear several times throughout the Mario series and its spin-offs. When someone is looking at them, they cover their face and become transparent. However, when they turn around, the Boo uncovers their face to reveal an evil grin and advances on them. When they turn around again they instantly stop and cover their face again. Sometimes a larger Boo will have several smaller Boos following it. The smaller Boos are at first concealed behind the regular Boo. To defeat a Boo, Yoshi must fire and egg into a wall so it will reflect into a Boo.

In Super Mario World 2: Yoshi's Island, Yoshi had to fight a boss called Bigger Boo, who was a giant Boo.

In Yoshi's Island DS, the first boss in World 2 was Hector the Reflector, a boss similar to a Big Boo.

In Yoshi's New Island, Boo is yet again an enemy and even has his own stage like in previous Yoshi games, "Beware the Boo Brigade"
Boo with a tounge low