Microbes have always been thought to be a critical inhabitant of the colobine gastrointestinal tract given their ability to break down the fiber and toxins often associated with a leafy diet. In this chapter, we summarize current knowledge of the colobine gut microbiome and propose topics for future study. We begin by describing the composition and function of the colobine hindgut microbiome as it compares to other primates, as well as ruminants. Next we explore the microbiome as a tool for understanding and assessing colobine health outcomes both in captivity and in response to degradation of habitats in the wild. Finally, we outline some best practices and caveats for future studies of the colobine gut microbiome more generally. Although there is currently a dearth of studies describing colobine host-gut microbe interactions, existing data suggest that the potential for the gut microbiome to influence colobine health, ecology, and evolution is great.