The variety of different body matrices that can be analyzed to determine the presence or absence of different psychoactive substances is extensive, ranging from semen to cerumen. There are, however, practical limitations to the extent to which different biological samples can be used, and the mechanism of collection and supervision of samples are critical to the procedure. This chapter focuses on those biological samples that are commonly used for testing within various drug treatment settings, namely, urine, saliva, blood, and hair. Urinalysis is routinely used in hospitalbased services, blood in forensic environments, hair analysis for medicolegal cases, and saliva tests have been used in the prison services and outreach units.