Disorders determined wholly or in part by genetic factors constitute a substantial number of human diseases. This realization has grown during the past 2 decades with the recognition of many specific heritable conditions and the identification of familial risk factors for common disorders. New technologies, such as fetal visualization, chorionic villus sampling, molecular cloning methods, and gene transfer technology, provide a framework for dealing with genetically determined illness in unprecedented ways. Several current and potential applications of these methods are examined, as is the use of restriction fragment length polymorphisms to survey the variability within the genome and to generate markers permitting the prospective detection of genetic disorders. The promise and limitations of chorionic villus biopsy sampling are considered for early prenatal diagnosis. The future of gene therapy in hereditary diseases is examined, and some of the substantial social and ethical considerations engendered by these new developments are explored.