This chapter examines some of the existing and new challenges for forensic toxicologists analysing biological fluids for drugs, alcohol and poisons. In recent years there have been impressive advances in instrumentation, which have enabled toxicology laboratories to make more sensitive measurements and analyse new types of samples, drugs, inhalants and also some endogenous biomarkers. This has brought new interpretation challenges, including analytical uncertainty, and has all had to be achieved whilst working to updated accreditation and quality standards.