Recent advances in molecular biology have rendered these sophisticated basic science techniques, formerly confined to the research laboratory, directly applicable to clinical medicine. Of all medical specialties, pathology and laboratory medicine are in the best position to immediately take advantage of these developments. The most exciting clinical applications are thus far largely diagnostic rather than therapeutic-the pathology laboratory (both clinical and anatomic) has primary access to the target specimens, and pathology is traditionally the medical specialty most intimately concerned with defining the fundamental nature of disease. The techniques discussed in this review are those that will carry this tradition of pathologic inquiry to the diagnosis and description of disease at the most fundamental level of all-the molecular one.