Aims. To examine a range of biological indicators of illicit drug use, including blood, urine, hair and saliva, addressing both technological and practical issues relating to their application and interpretation. Methods. The review process involved an examination of key reference texts and literature from the scientific fields of analytical and clinical toxicology. Findings. Urine remains the biological tool of choice for qualitative detection of illicit drug use in a clinical setting, while quantitative accuracy remains strictly the domain of blood. The growing sophistication of laboratory analysis may additionally make possible the routine use of hair sampling which can provide a much longer time frame for assessment. Breath, saliva, sweat or breast milk remain possibilities in the future. Conclusions. Accurate interpretation of the screening tests within a clinical setting alongside other relevant information remains the key to the usefulness of any test.