Infanticide, the killing of infants, is well documented in chimpanzees. However, despite multiple reports of this behaviour, no comprehensive study has attempted to explicitly test the various different hypotheses for its adaptive function in this species. In this thesis, I use records from the Sonso community of chimpanzees in the Budongo Forest, Uganda, to address the question of the function of infanticide. While infanticide has been recorded in several communities of chimpanzees, rates vary dramatically. In order to shed light on this variation, this thesis investigates risk factors for infanticidal behaviour and counter-strategies by parents. I found that the majority of infanticides adhere to the predictions of the sexual selection hypothesis for male committed infanticide, that instability in the male hierarchy is a key risk factor, and that mothers, but not fathers employ counterstrategies to protect their infants. Avoidance of potentially infanticidal individuals appears to be a key maternal counterstrategy to infanticide. Mothers are responsive to the varying risks posed by different individuals and how they change over time and they selectively avoid high risk individuals, both at the between and within party level.