This is the first of two papers which review issues concerning complementary medicines. The first reviews the extent of use of complementary medicines, and issues related to the regulation and pharmaceutical quality of these products; the second considers evidence for the efficacy of several well-known complementary medicines, and discusses complementary-medicines pharmacovigilance. The term complementary medicines describes a range of pharmaceutical-type preparations, including herbal medicines, homoeopathic remedies, essential oils and dietary supplements, which mainly sit outside conventional medicine. The use of complementary medicines is a popular healthcare approach in the UK, and there are signs that the use of such products is continuing to increase. Patients and the public use complementary medicines for health maintenance, for the treatment or prevention of minor ailments, and also for serious, chronic illnesses. The pervasive use of complementary medicines raises several concerns. Many of these arise because most complementary medicines are not licensed as medicines, and therefore evidence of quality, efficacy and safety is not required before marketing. The regulatory framework for herbal medicines and dietary supplements is currently under review. A new system for registration of traditional herbal medicines will ensure that marketed products meet standards for quality and safety. At present, the pharmaceutical quality of many complementary medicines is a cause for concern.