Impulsivity and compulsivity are symptoms that cut across many psychiatric disorders. Some conditions with impulsivity as a prominent feature have already been discussed, including mania (Chapter 4); attention deficit hyperactivity disorder (ADHD; Chapter 11), and agitation in dementia (Chapter 12). Several other disorders in which impulsivity and/or compulsivity are core features are discussed in this chapter. Full clinical descriptions and formal criteria for how to diagnose the numerous known diagnostic entities discussed here should be obtained by consulting standard diagnostic and reference sources. Here we emphasize what is known or hypothesized about the brain circuits and neurotransmitters mediating impulsivity and compulsivity, and how engaging neurotransmitters at various nodes in impulsivity/compulsivity networks can result in successful psychopharmacological treatments.