Since the first report of infant killing in Hanuman langurs (Presbytis entellus) by Sugiyama (1965b), numerous hypotheses regarding the bio­ logical function and/or causation of infanticide in langurs have been developed (reviewed in Hausfater and Vogel, 1982). Most discussion, however, has concentrated on the sexual selection hypothesis (Hrdy, 1974). The aim of this chapter is to test this hypothesis as well as its basic assumptions against field data from our long-term study (19771982) of langurs at Jodhpur (Rajasthan), India.