Herbal medicine is used worldwide either as a sole treatment method or as part of a comprehensive treatment plan alongside orthodox methods of diagnosis and treatment. A survey reported that, in the USA, nearly one-sixth of women took at least one herbal product in 2000. Despite their widespread use, numerous reports show that the herbal products available to consumers are of variable quality. This disparity in quality of herbal preparations can be attributed to the fact that their production is complicated. To produce high-quality herbal products, attention must be paid to, among others, phytochemical variations due to plant breed, organ specificity, stages of growth, cultivation parameters, contamination by microbial and chemical agents, substitution, adulteration with synthetic drugs, heavy metal contamination, storage and extraction. This review focuses on organ specificity, seasonal variations, the effect of drying and storage, and the extraction of phytochemical constituents. Special emphasis is placed on the four most frequently used herbal products in the USA: echinacea, Ginkgo biloba, ginseng and St John's Wort.