This chapter reviews Frisch's work, considers its application to the understanding of population dynamics in hunting and gathering populations, and looks at the available evidence from the !Kung on some of the relevant variables. Frisch began the series of studies that led to the critical fatness hypothesis with the question of the determinants of menarche in young women. Frisch is aware that although the critical fatness hypothesis may not account for much of the variation in fertility, it offers a promising means to advance our understanding of the determinants of fertility control in hunting and gathering societies. The chapter presents data on the distribution of height and weight by age among the Kung women, and looks at changes in weight for individual women at particular points in the reproductive process. Frisch's critical fatness hypothesis is relatively new and untested. The hypothesis requires and deserves a full-scale test in populations of women who do not practice contraception.