During the past two decades research on the testis has created a vast number of publications. New techniques, such as cell and gene clon­ ing, germ cell manipulation and transplantation, as well as gene ex­ pression, have surpassed the classical approaches that involved studies on receptor activation and endocrine, paracrine and aurine regula­ tory mechanisms. Due to the expansion of this field of research it has become increasingly difficult to integrate the large number of observations and relate them to the physiology and pathophysiology of the tes­ tis. More emphasis must, therefore, be put on the integration of isolated findings and the construction of the total picture. In order to stimulate this specific mental activity we have named this book Testicular Tan­ gram. We hope that the chapters in this book will be used as pieces of the testicular puzzle and that the reader enjoys composing new shapes. The chapters of this book are representative of the plenary lectures presented at the I ih European Workshop on Molecular and Cellular Endocrinology of the Testis held in Doorwerth The Netherlands, from April 6 - I 0, 2002. Publication of this book has been made possible by the generous support of the Ernst Schering Research Foundation. We would also like to thank the distinguished scientists, who have provided us with their excellent manuscripts well before the start of the Workshop. We acknowledge the help of Dr. U. -F. Habenicht, Dr. M.