This report, prepared for the UK House of Commons Select Committee on Science and Technology, presents the results of four case studies examining the evidence base for the classification of illegal drugs in the context of the Misuse of Drugs Act 1971. It identifies the main evidence on the selected drugs (amphetamines and ecstasy, cocaine, magic mushrooms and cannabis) and examines the use of that evidence in classifying each drug. The report also briefly examines the classification systems in three other countries (the USA, Sweden and the Netherlands), giving information on the penalties and treatment regimes associated with them.