Starting from the description of the structure, number and meiotic behaviour of chromosomes, the evolution of the economically important Nicotiana species is analyzed. Flowering, expected fertility, incompatibility and male sterility, seed development, as well as germination, are used as criteria to outline the principles of Nicotiana breeding. The chapter entitled"Sortenz"uchtung"(variety breeding) includes a survey of the development of varieties, the breeding material available and that to be established. The application of sterile culture techniques in Nicotiana breeding is discussed. The sentation of the different breeding methods applied includes a compilation of released tobacco varieties. A bibliography is appended (279 ref.).