Although the brain was once seen as a rather static organ, it is now clear that the organization of brain circuitry is constantly changing as a function of experience. These changes are referred to as brain plasticity, and they are associated with functional changes that include phenomena such as memory, addiction, and recovery of function. Recent research has shown that brain plasticity and behavior can be influenced by a myriad of factors, including both pre- and postnatal experience, drugs, hormones, maturation, aging, diet, disease, and stress. Understanding how these factors influence brain organization and function is important not only for understanding both normal and abnormal behavior, but also for designing treatments for behavioral and psychological disorders ranging from addiction to stroke.