Blood, oral fluid (saliva), urine and hair are the most commonly used biological matrices for drug testing in epidemiological drug research. Other biological matrices may also be used for selected purposes. Blood re-flects recent drug intake and may be used to assess impairment. Oral fluid reflects drug presence in blood and thereby also recent intake, but drug concentrations in this matrix cannot be used to accurately estimate con-centrations in blood. Urine reflects drug use during the last few days and in some cases for a longer period, but does not indicate the dose size or frequency of use. Hair reflects drug use during several months, but is a poor matrix for detecting use of cannabis. If using a single drug dose, this can be detected in blood and urine if the sample is taken within the detection timeframes, in most cases also in oral fluid. Single drug use is most often insufficient for producing a positive test result in a sample of hair. For cocaine and amphetamine, weekly use may be needed, while for cannabis a positive result is not guaranteed even after daily use. Refusal rates are lowest for oral fluid and highest for blood and hair samples. The analytical costs are lowest for urine and highest for hair. Combined use of questionnaires/interviews and drug testing detects more drug use than when using only one of those methods and is therefore expected to give more accurate data.