The howler monkeys (genus Alouatta monotypic in subfamily Alouattinae) are among the largest of the New World monkeys. Nine species are currently recognised. Previously classified in the family Cebidae, they are now placed in the family Atelidae.

Howler monkeys range in size from 56 to 92cm, excluding their tail which can be equally as long. Like many New World monkeys, they have prehensile tails. They have a lifespan of 15 to 20 years. These monkeys are native to South and Central American forests. They live in groups of usually about 18 individuals. Threats to howler monkeys include being hunted for food and captivity.

As their name suggests, vocal animal communication forms an important part of their social behavior. Specifically, the name comes from their distinctive loud barking whoop they make, which can be heard over considerable distances. (As correctly pointed out by William Henry Hudson in his novel Green Mansions, "howler" is a misnomer since the male's voice sounds rather like a powerful roar; female vocalisations sound like a pig's grunt). They have an enlarged basihyal or hyoid bone which helps them make their loud sound. They are considered the loudest land animal. Unlike other New World monkeys, both male and female howler monkeys have trichromatic colour vision.

These large, slow moving monkeys are the only folivores of the New World monkeys. Howlers eat mainly top canopy leaves, together with fruit, buds, flowers, and nuts. They move quadrapedally and do not brachiate, usually holding on to a branch with at least two hands or one hand and the tail at all times. Their prehensile tails are strong enough to support the monkey's entire body weight, although they seldom do so. The first 2 fingers of each hand are set apart and are opposable to the other three. They very seldom leave the trees. They rest about 80 percent of the time and are considered the least active of all monkeys.