Hybridization, or the process by which individuals from genetically distinct populations (e.g., species, subspecies) mate and produce at least some offspring, is of great relevance to understanding the basis of reproductive isolation and, in some cases, the origins of biodiversity. Natural hybridization among primates has been well documented for a few taxa, but just recently the genetic confirmation of hybridization for a number of taxa has produced new awareness of the prevalence of this phenomenon within the order and its importance in primate evolution. The study of hybridization of Alouatta pigra and A. palliata in Mexico was among the first to genetically confirm the current occurrence of hybridization in primates. Following this study, other reports of hybridization have shown that this phenomenon is more widespread among primates than previously anticipated. Within the genus Alouatta, there have been reports on the presence of hybridization between A. caraya and A. guariba in a number of contact zones in Brazil and Argentina, and various studies are currently ongoing in some of these sites to understand the extent and patterns of hybridization between these species. In this chapter, we evaluate the extent of hybridization in the genus Alouatta, revise the current knowledge of the genetic and morphological aspects of these hybrid systems, and identify future directions in the study of hybridization within this genus, to understand the possible implications of the hybridization process in the evolutionary history of howler monkeys.