Abnormal uterine bleeding (AUB) is any bleeding outside of the normal menstrual cycle, and it accounts for nearly one third of all visits to gynecologic providers annually. Disturbances to the timing and amount of uterine bleeding are multicausal, and vary in significance. The menstrual cycle is a self-perpetuating feedback loop of hormones generated by the hypothalamus, pituitary, and ovaries. This chapter addresses AUB in the order in which a clinical assessment progresses, beginning with pregnancy, then iatrogenic causes, systemic causes, pelvic or genital tract abnormalities, and finally anovulatory bleeding also called dysfunctional uterine bleeding (DUB) and amenorrhea. It achieves the process of diagnosis through history, and physical examination, augmented by laboratory tests and visualization. Interaction between the pelvic anatomy and the brain determines the success or failure of the menstrual cycle. Menstruation is controlled by an interacting system of hormones derived from the hypothalamus, the anterior pituitary, and the ovaries.