In almost all sexually reproducing organisms (including humans), males and females often deploy conflicting strategies to pass their genes on to the next generation. This is especially true in species where males and females mate with more than one individual. In an evolutionary sense, neither a father nor a mother “cares” as much about the fitness of their partner as they do about the fitness of their offspring. I will discuss the impacts this conflict has on the evolution of genitalia and seminal fluid, with special consideration of humans.