'Trease and Evans' is an encyclopedic reference work on pharmacognosy - the study of those natural substances, principally plants, that find a use in medicine. Its popularity and longevity stem from the book's balance between classical (crude and powdered drugs' characterization and examination) and modern (phytochemisty and pharmacology) aspects of this branch of science, as well as the editor's recognition in recent years of the growing importance of complementary medicines, including herbal, homeopathic and aromatherapy.