About 21 years ago prenatal diagnosis became part of the physician's diagnostic armamentarium against genetic defects. My first monograph in 1973 (The Prenatal Diagnosis of Hereditary Disorders) critically assessed early progress and enunciated basic principles in the systematic approach to prenatal genetic diagnosis. Six years later and under the current title, a subsequent volume provided the first major reference source on this subject. The present second (effectively third) edition, which was urged in view of the excellent reception of the two earlier volumes, reflects the remarkable growth of this new discipline and points to significant and exciting future developments. Notwithstanding these advances, the use of the new tools and techniques for the benefit of at-risk parents has taken many more years than most anticipated. Key factors have been the lack of teaching of human genetics in medical schools in the preceding decades and the difficulty of educating practicing physicians in a new scientific disci­ pline. Even today the teaching of genetics in medical schools leaves much to be desired and this will further delay the introduction of newer genetic advances to the bedside.