Howler monkeys are the largest among the monkeys in the New World and they are distributed across the tropical forests of Central and South America. A total of 10 different species have been recognized; they include the red howler, red-handed howler, black-and-gold howler, Coiba Island howler, red-and-black howler, Guyana red howler, Bolivian red howler, black howler, Amazon black howler and mantled howler. The howlers have amazing prehensile tails and its fun to watch them feeding while hanging upside down by their tails. The author, a field primatologist with three decades of field research experience from Asia, Africa and South America, has studied two species of howler monkeys such as the red howlers in Venezuela and Trinidad, and the black-and-gold howlers in Argentina. In this monograph, the author has synthesized his long-term field research and presented data on ecology, social behavior and conservation. This monograph can serve as an introductory and reference text on primatology and animal behavior related courses in colleges, universities and institutes of higher learning globally.