Menopause, the final cessation of menstrual cycling, occurs when the pool of ovarian follicles is depleted. The one to five years just prior to the menopause are usually marked by increasing variability in menstrual cycle length, frequency of ovulation, and levels of reproductive hormones. Little is known about the mechanisms that account for these characteristics of ovarian cycles as the menopause approaches. Some evidence suggests that the dwindling pool of follicles itself is responsible for cycle characteristics during the perimenopausal transition. Another hypothesis is that the increased variability reflects "slippage" of the hypothalamus, which loses the ability to regulate menstrual cycles at older reproductive ages. This paper examines the underlying cause of the increasing variability in menstrual cycle length prior to the menopause. A model of ovarian cycles is developed, based on the process of follicular growth and depletion. Under this model, the follicular phase of each menstrual cycle is preceded by an inactive phase, a period of time when no ovarian follicles have left the resting state and begun secreting steroids in response to gonadotropin stimulation. The model makes predictions about the variability in menstrual cycles across the reproductive life span based on the size of the surviving pool of ovarian follicles. We show that the model can explain several characteristics of the perimenopause in humans and macaques and illustrate how the model can be applied to research on the biological and cultural correlates of the timing of menopause.