Orangutans (genus Pongo) represent the extreme of many biological parameters. They are the largest priniarily arboreal mammal, have the longest interbirth interval, are the most solitary of the ·diurnal primates., and are the most sexually dimorphic of the great apes. They range over large areas and have diffuse communities. Additionally, adult male orangutans come in two morphologically distinct types, an unusual phenomenon known as bimaturism. This intriguing suite of features has proven difficult for researchers to fully understand. The semi-solitary lifestyle of orangutans, combined with their slow life histories and large ranges, means data accumulate slowly. Also, rapid habitat destruction in Southeast Asia has left only a handful of field sites currently in operation (Figure 18.1 and Table 18.1). Despite these difficulties, long-term behavioral studies and recent genetic and hormonal data now allow us to explore some of the most theoretically interesting questions about this endangered great ape. Here, we summarize our current state of knowledge about wild orangutans and discuss advances in our understanding of one distinctive orangutan behavior-forced copulations.