Positron emission tomography (PET) and single photon emission computerized tomography (SPECT) brain imaging techniques are powerful tools for the investigation of human neurochemistry and psychopharmacology. These versatile in vivo methods can be used to measure brain receptor populations in psychiatric illness, to determine receptor occupancy and tissue pharmacokinetics of psychotropic drugs and to examine the functional effects of drug-induced neurotransmitter manipulation. This chapter reviews recent innovative basic and clinical studies, of relevance to psychiatry, and discusses methodological developments that extend the use of these techniques.