Psychoactive or behaviorally active drugs are substances that alter internal and external behavioral processes including activity levels, moods and feelings. As a result of these changes, while some of these substances can lead to compulsive drug use and drug addiction, others are used to manage neuropsychological disorders. In both cases these drugs do not produce unique behavioral or neurological effects. Their behavioral activity results from modifying existing neuronal systems. To understand the actions of abused drugs on the brain, one must have an understanding of the functions that brain cells serve in the expression of behavior in general. This article focuses on information that will assist readers in understanding the biological basis of drug actions on the brain, and particularly the actions of commonly abused drugs. First, the general classification of brain cells will be discussed, followed by a discussion of brain...