The XIIth North American Testis Workshop was held in Tampa, Florida, April 13 to 16, 1993. The program consisted of 23 invited lectures and 120 poster presentations. In keeping with the tradition of the Testis Workshop, every attempt was made to emphasize recent develop- ments in the study of endocrine and gametogenic functions of the male gonad. Although the main emphasis, reflected in the title of the pro- ceedings, was on the function and control of somatic cells in the testis, one session was devoted to germ cells and one to the studies of male reproductive development and function. The last session of the workshop was included in the program of the Postgraduate Course of the American Society of Andrology, which held its annual meeting in Tampa imme- diately following the workshop. This volume consists of 21 chapters prepared by the speakers and 8 chapters contributed by presenters of the posters. The Program Committee consisted of Drs. Nancy Alexander, C. Wayne Bardin, Andrzej Bartke (Chairman), Kevin J. Catt, Claude Desjardins, Maria Dufau, Michael Griswold, Norman Hecht, Aaron Hsueh, Dolores J. Patanelli, Bernard Robaire, and Richard Sherins. The workshop was generously funded by Sero Symposia, USA, which also agreed to handle registration, local arrangements, and publication of this volume. On behalf of the Program Committee and all participants, I would like to thank Dr. Bruce Burnett of Sero Symposia, USA and his staff for their efforts and support in every aspect of the organization of the workshop.