As part of criminal investigations, toxicological analysis retains an important role but the focus has changed in recent years. Public concern over the use of illegal psychoactive drugs, plus heavy reliance on police agencies to enforce drug laws, have created a new and enhanced role for toxicological testing over the past 20 years. Such testing has multiple uses in the criminal justice system, but the one of most immediate concern to practitioners is the detection of illegal drug use by persons in custody. Within this context, drug testing can occur at several points in the processing of offenders. The evolution of drug testing technology is reviewed, with emphasis on chromatography and immunoassays. The growth of interest in drug testing is also examined in terms of the trend toward more aggressive drug testing, pretrial monitoring of arrestees, and post-conviction drug testing. Emerging drug testing technologies are noted, including hair analysis, sweat patches, saliva testing, and multimodal testing with the ion mobility spectrometer. Policy challenges and the future of drug testing are discussed.