Molecular pathology, defined broadly as the use of nucleic acid probes to diagnose and study disease, is an emerging discipline of growing importance and promise. Utilizing the principles of nucleotide base-pairing for specific hybridization between a DNA or RNA probe and its complementary target sequence, molecular diagnostic techniques are finding ever-increasing applications across the entire spectrum of human disease. These include infectious diseases (using DNA probes for viruses, bacteria, and parasites), neoplastic diseases (through detection of gene rearrangements, tissue-specific gene transcription, and oncogene activation), hereditary diseases (by screening for specific mutated genes or linked DNA polymorphisms), and the differentiation of individuals from one another by "DNA fingerprinting" (for purposes of donor recipient identification in transplants, paternity testing, or forensic investigations). This review surveys the current applications in each of these areas, along with the most important techniques now being used: Southern blotting, in situ hybridization, and the polymerase chain reaction. Finally, the impact of these powerful new methodologies on the entire field of diagnostic pathology is discussed.