The first edition of this book, published in 1979, was found useful by many stu dents and was well received by the scientific community. Since the book was first written, human genetics has undergone dramatic developments, mainly due to the introduction of new concepts and techniques from molecular biology. Con comitantly, "basic" scientists have become increasingly interested in problems of human genetics. More than 700 human genes have been mapped, genes of previ ously unsuspected complexity -such as the gene for factor VIII - have become known, and the structure of noncoding DNA sequences is being analyzed with the aim of understanding gene regulation. DNA diagnosis is being rapidly intro duced into medical genetics. All this, as well as the extensive progress in most other fields of human and medical genetics, had to be considered in the prepara tion of this second edition. The book has been extensively revised and rewritten. A substantial new section dealing with gene and chromosomal structure at the molecular level has been added. The newer knowledge of molecular genetics has been incorporated, and the conceptual and practical contribution of DNA methods (for example in the hemoglobinopathies and in some other diseases) is discussed. Many new figures and tables have been added, and some illustrative material has been replaced. We have read carefully the many friendly and sometimes flattering reviews of the first edition.