Biological, psychological, and social factors that may individually and interactively affect the menstrual cycle and related disorders have been subjected to considerable research. This chapter reviews what is known about the menstrual cycle, premenstrual syndrome, and, particularly, the role of caffeine in the overall cycle and the disorder. Researchers have examined the effect of caffeine on the output of various reproductive hormones, such as follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone. The chapter focuses on the physiology and psychology of the menstrual cycle as these interact with the presence of caffeine in the body. The menstrual cycle is divided into three phases: the menstrual phase; the follicular phase; and the luteal phase. The chapter also focuses on the question of how caffeine affects the menstrual cycle and how its pharmacokinetics is affected by changing menstrual physiology. It is concerned with whether or not caffeine consumption is related to the prevalence and severity of premenstrual symptoms.