Prenatal diagnosis (PD) is defined as a relatively new branch of medical genetics. Its major goal is to study human genome functions in normal and abnormal development, embryonic-maternal interactions. Its practical application is confined to elaboration of new effective ways to prevent hereditary diseases and probably somewhat later their specific corrections. Most urgent problems of prenatal diagnosis are outlined and briefly discussed. They include RFLP analysis in high risk families and organization of DNA data banks; wide application of polymcrase chain reaction; search for new informative RFLP sites; molecular analysis of mutated genes and their primary effects. Urgent problems in prenatal diagnosis of chromosomal diseases include application of chromosome specific DNA probes; the studies of aneuploidy origin and chromosomal mosaicism detection; ellaboration of non-invasive methods for karyotyping and sexing of the human embryos. Possible ways for corrections and treatment of hereditary diseases in man are briefly discussed.