Blood flow, metabolism, and structural imaging studies suggest altered neural circuits in major psychiatric disorders including mood disorders, schizophrenia, and obsessive compulsive disorder. Neuroreceptor mapping studies have identified serotonergic abnormalities in mood disorders and dopaminergic abnormalities in schizophrenia. Further imaging applications have involved development of new positron emission tomography (PET) tracers that may identify abnormalities in peptide neurotransmitter systems such as corticotrophin releasing factor or substance P. Finally, PET can play an important role in quantifying the relationship between receptor occupancy, drug blood levels, oral dose and therapeutic outcome. In that way PET scanning can contribute to both therapeutics and to drug development by more rapid identification of the likely therapeutic dose range compared with conventional parallel group dose comparisons or dose ranging studies.