Preparatory work for this monograph started in 1988 and 1989. The motive was provided by certain incongruities in the illustra- tion of descensus testis for the chapter "Entwicklung der Genital- organe" in the book Humanembryologie. The evoked discussion was reason enough for our own re- search on the phenomenon and uncertainties of testicular des- cent in human. The basis ofthese investigations was the collection of human embryos founded by Prof. Hinrichsen in 1970 at the Department of Anatomy and Embryology, Ruhr-University Bo- chum. The preparation of the material used in this paper has been done with the help of many staff members of our department to whom we feel obliged. We are especially indebted to Prof. Hin- richsen who prepared the foetuses and took the first steps of this project. We wish also to acknowledge Dr. Heinz Jiirgen Jacob for careful fixation and microdissection of specimens intended for scanning electron microscopy. Weare also grateful to Mrs. Vera Mannheim for her involvement and her sensitivity in making scanning electron micrographs, and Mrs. Antje Jaeger for provid- ing an expert and skilful photographic technique especially in the setting up of micrographs. Our thanks are also due to Mrs. Marion Kohn for skilful technical assistance and to all those technical assistants who worked in our department during the past 25 years and who were engaged in providing the serial sec- tions. The English text has been thoroughly revised by Prof. Dr.