The present study was originally intended to be a taxonomic revision of the genus Pygeum (Rosaceae). This genus was always considered to be closely related to, but different from Prunus. However, the delimitation of Pygeum against Prunus subg. Laurocerasus appeared to be extremely difficult, and after careful consideration I was compelled to reduce Pygeum to Prunus. This reduction is the most radical taxonomic novelty in the present paper (Chapter 3). A new subdivision of subg. Laurocerasus, now enriched with Pygeum, was inevitable. Three sections are distinguished here, viz.: a. sect. Laurocerasus, containing the Eurasian and tropical Asiatic species which were already considered by older authors to belong to the (sub)genus Laurocerasus, and moreover the two African species of the former genus Pygeum; b. a still unnamed section containing the tropical and subtropical American species of the subgenus; c. sect. Mesopygeum, containing the large majority of the species of the former genus Pygeum.