Melanosomes are specialized subcellular organelles in which melanin is synthesized and deposited. Electron microscopic, cytochemical, genetic, and biochemical evidence all support the contention that melanosomes are specialized lysosomes. The relationship of melanosomes and lysosomes provides a framework in which to understand the pathogenesis of disorders such as the Chediak-Higashi syndrome, allows the testing of hypotheses for the trafficking of proteins to melanosomes, and has important implications for the chemistry of melanization and the potential pharmacologic manipulation of that process. In addition, the lysosome-like nature of melanosomes may provide insight into the processing and presentation of melanosomal antigens by melanoma cells.