Other chapters in this textbook address the questions of how psychotropic medications affect the brain to reduce the severity of the clinical features and symptoms of psychiatric disorders and to produce the side effects that frequently accompany their administration. Appropriately, much attention has been directed toward the neurotransmitter systems that are the targets of these medications. A potential consequence of this emphasis is the idea, in its simplest form, that an excess or deficit in the functional activity of a given neurotransmitter is the pathophysiological basis for the clinical features of interest. Although variants of this view have been very useful in motivating investigations of the molecular underpinnings and biochemical features of neurotransmitter systems and in spurring the development of novel psychopharmacological agents that influence these systems, in the extreme case this perspective tends to consider a given psychiatric disorder as the consequence solely of the postulated disturbance in a neurotransmitter system.