Life history theory (LHT) is a branch of evolutionary theory aimed at explaining differences in the age schedules of growth, reproduction, and mortality . LHT proposes that humans, like other species, face trade-offs in how to optimally allocate the limited resources of time and energy among growth, reproduction, and maintenance (i.e., immune function, survival, longevity). Optimal allocation between these demands is expected to depend on individual and ecological parameters that influence resource production, mortality, and other factors. There are inherent trade-offs between somatic versus reproductive effort , current versus future reproduction, mating versus parental effort, and quantity versus quality of offspring. The theory posits that there have been long-term evolutionary pressures to balance energetic effort, and channel resources toward behaviors that maximize expected reproductive success across the life span. Life history strategies of energy allocation interact with the environment to fundamentally determine rates of growth, age of reproductive maturation, the pace of reproduction, reproductive senescence (particularly in humans), and life span. Physical activity is particularly well suited to be examined from a life history perspective because it both influences and is influenced by life history parameters (such as age of reproductive maturation, survival, and longevity) and is central to both energy expenditure and traditionally, energy acquisition. This chapter examines the costs and benefits of allocating energy to physical activity in humans.