It seems self-evident that the brain controls behavior but does behavior also "control" the brain? This chapter examines evidence that behavior can and does influence specific aspects of brain structure and function. Evidence for such influence is easily obtained on an evolutionary time scale, since the selective forces found in the ecological niche of the animal are typically reflected in its sensory and motor activities as well as its body shape and behavioral habits. Similarly, during development, there is ample evidence that the behavior acts in concert with the environment to establish structural changes in the brain that last a lifetime. Perhaps most surprisingly there is now evidence that social behavior can cause changes in the brain in adult animals and that these changes are reversible. The changes caused by behavioral interactions can be dramatic and typically are related to reproductive behavior. Understanding the mechanisms responsible for dynamic changes in the nervous systems of adult animals is a major challenge and the discovery that it can occur may lead to insights about other systems where behavior sculpts the brain.