Products of secondary metabolism from fermentation and cell culture are mainly pharmaceuticals, ranging from traditional antibacterials like ß-lactams, new generation compounds like dalbaheptides and lantibiotics to immunomodulators and antitumor drugs. The major fields of application of these low-volume/high-price products are human and veterinary medicine as well as plant protection. In this volume functionally based screening approaches and the applications of molecular genetics in the discovery and process improvement of compounds are emphasized. Along with the exploitation of new sources the growing understanding of biosynthetic processes permits the identification of the genetic backgrounds.