Human gene mapping is concerned with the assignment of individual genes to specific parts of chromosomes. The rapid progress in this field has shown the importance of genetics to human biology and to clinical medicine. Several approaches are available for expanding the human gene map. The gene dosage approach was used in the mapping of the esterase D and retinoblastoma locus on the long arm of chromosome 13. The principles behind kindred linkage analysis and the application of somatic cell hybridization techniques are examined in relation to human gene mapping. Recent data on the steroid sulfatase gene suggest that not all X-linked genes are susceptible to inactivation. Several clinical applications of human gene mapping and linkage information are presented. With the application of current molecular techniques, the human gene map may be substantially completed by the end of the century.