This third edition of Genetic Counseling has been thoroughly revised to reflect current practice. In particular, the chapter on prenatal diagnosis (Chapter 10) has been largely rewritten and considerably expanded, reflecting the rapid develop ment in this field and its increasing medical importance. This chapter provides a detailed description of the alpha-fetoprotein test and a brief description of ultrasonography for the diagnosis of morphologic defects. Further, we discuss fetoscopy and the sampling of fetal blood with the aid of a fetoscope. In this as in past editions, these descriptions are not meant as working instructions for actual practice, but rather as background for the general practitioner who is dealing with the problems of genetic counseling. We have added a chapter (Chapter 7) on the use of conditional probability (Bayes Principle) for the calculation of more exact specific risk figures. It is true that the daily practice of medicine sees far fewer situations in which these methods can reasonably be applied than some theoreticians like to think. However, the usefulness of these methods has recently increased, especially in cases of X-chromosomal recessive diseases where the termination of a preg nancy, if the fetus has been ascertained to be male and if the mother is most probably heterozygous, has become more commonplace. With such a trend, the degree of probability must be determined as exactly as possible.