There is burgeoning interest in the costs of mating and in how this leads to sex differences in the optimal mating rate. Here, we attempt to look beyond an exclusive focus on matings to examine the fundamental conflicts that arise out of the fact that sexual reproduction involves investment in offspring by typically unrelated individuals. Conflicts between mates stem from the ever present potential for individuals to reduce their investment relative to that of their sexual partner in order to seek alternative reproductive opportunities elsewhere. All aspects of life histories, the partitioning of time and energy into growth, survival and reproduction, are inextricably caught up in, and moulded by these conflicts of interest, the most profound expression of which is the differentiation between male and female types. This differentiation creates a new set of conflicts because alleles with positive effects on fitness when expressed in one sex may have negative fitness effects in the other. This type of conflict may influence the evolution of separate juvenile and adult life stages as in metamorphosis, although a major outstanding question is why sex limitation does not evolve more rapidly. We review the impact of conflicts of interest between males and females on all aspects of life history, revealing at least as much evidence for sexual conflict over life history traits such as investment in gametes and parental care as there is for conflicts over matings.