Primary acquired melanosis (PAM), a disease that affects mostly middle-aged white patients, is predominantly a proliferative condition of the melanocytes that normally populate the conjunctival epithelium. Primary acquired melanosis without atypia (low risk for the development of melanoma) is typically created by increased numbers of melanocytes restricted to the basilar region of the epithelium without nuclear hyperchromasia or prominence of the nucleoli. Primary acquired melanosis with atypia, a formal precursor of melanoma, is characterized by the proliferation of small polyhedral cells, spindle cells, large dendritiform melanocytes, or epithelioid cells that may: remain restricted to the basilar region (basilar nests); form nests at all levels of the epithelium; spread individually to all levels of the epithelium (pagetoid extension); or proliferate in a sheet-like fashion approximating a melanoma in situ. Lesions composed of epithelioid cells or exhibiting intraepithelial pagetoid extension have, respectively, a 75 or 90% chance of eventuating in invasive melanoma. Primary acquired melanosis in an adult should not be confused with "a junctional nevus," which is almost always restricted to childhood. Invasive melanomas measuring less than 0.8 mm in thickness tend not to be associated with metastases; the tumor cells may be small polyhedral (in which case confusion with a compound nevus often arises), epithelioid, spindled, or ballooned. Nodules composed of spindle cells in part or in toto tend to have less metastatic potential at a given thickness measurement than comparable nodules composed of epithelioid or polyhedral cells. The clinical features, electron microscopic findings, and biologic principles underwriting clinical management are also presented.