The term ocular melanoma refers to a heterogeneous group of cancers of melanocytic origin. The precursor of most cases of conjunctival melanoma is known to ophthalmologists as primary acquired melanosis. This condition passes through well-defined stages of tumor progression. Although tumor progression is not obligatory, as a conjunctival melanocytic lesion acquires new biologic properties it is more likely to progress further. Although junctional nevi are seldom encountered beyond childhood and primary acquired melanosis usually develops in middle-aged individuals, these two conditions may be histologically indistinguishable. Most junctional nevi eventually show evidence of differentiation, whereas nearly half of the cases of primary acquired melanosis with atypia progress to melanoma. Therefore, it is possible that aging may modulate the capability of certain clonal proliferations to differentiate. Uveal melanocytes normally reside in mesenchyme, so that the traditional histologic criterion for establishing the diagnosis of most melanomas--breach of an epithelial basement membrane--does not apply. Because uveal melanomas are not easily accessible to incisional biopsy (without disruption of vision), only two points in the spectrum of tumor progression are defined clinically: nevus and melanoma. Experimental evidence suggests that a spectrum of atypical melanocytic proliferations separates benign nevi from melanomas capable of generating metastases. Unlike conjunctival melanomas that spread first to regional lymph nodes, choroidal and ciliary body melanomas preferentially spread first to the liver and are examples of organ-specific metastases.