The first two editions of this book, published in 1979 and in 1986, were well re ceived by the scientific community. Translations into Italian, Japanese, and Rus sian suggest that this book was regarded useful in many parts of the world. Mean while, human genetics has seen dramatic developments, and the "molecular revo lution" has attracted thousands of scientists, including many molecular biologists, to this field. About 3700 human genes have already been mapped to chromosomal sites. Many such genes have been cloned, and the various mutations causing dis ease have been identified. Novel mutational mechanisms such as expanded trinu cleotide repeats have been discovered in conditions such as Huntington''s disease and the fragile X syndrome of mental retardation. Gene action now can often be elucidated by studying the pathway from gene to phenotype following positional cloning rather than working in the opposite direction, as was customarily done be fore the tools of "new genetics"were available. In an increasing number of genetic diseases, the pathogenic mechanisms have been elucidated with positive conse quences for prevention and treatment. It therefore became necessary to rewrite al most completely major portions of this book. These developments are now making genetics arguably the leading basic science for medicine, as well as a recognized medical speciality. But all these changes do not mean that the entire framework of human genetics had to be reconstructed.