Publisher Summary This chapter reviews neuroanatomy and neurophysiology in order to help understand the relationship between the brain changes associated with various psychological disorders and the specific changes in mood, cognition, and behaviors that may be expected when psychoactive compounds act on the brain and alter brain function. Since psychopharmacology is the study of the mechanisms, effects, and use of substances that act on the brain, it is necessary to be familiar with the structure of the brain, as well as principles of brain function, in order to understand the action of psychoactive compounds. As the knowledge of the brain increases, one is becoming more aware of the complex nature of brain changes that underlie many psychological disorders. In some cases (for example, depression), the structure of the brain may change, and some parts of the brain may be larger or smaller.