In order that a new chemical entity may be marketed as a drug its pharmacological effects must first be evaluated using animals. Its bioavailability must also be determined and toxicological studies are performed to assess its acute and chronic toxicity, mutagenicity, effects on fertility, perinatal effects, teratogenicity and carcinogenicity. If the results of efficacy and toxicity studies in animals are favourable, the drug is then tested in a few human volunteers, followed by pilot studies and large-scale clinical trials. Regulations that control how these trials are performed are considered. The licensing of drugs in the UK, USA and Europe is compared.