Most genes are expressed from two alleles, one maternal and the other paternal. The term "genomic imprinting" refers to a genetic phenomenon which produces some interesting exceptions to this rule. Genes which are subject to imprinting are molecularly marked before fertilization such that they are transcriptionally silenced at one of the parental alleles in the offspring. A growing body of evidence implicates genomic imprinting in the pathogenesis of certain human genetic diseases, inherited tumor syndromes, and sporadic tumors. This review discusses examples of imprinting, theories as to why the phenomenon exists, possible molecular mechanisms of imprinting, and our current understanding of the role of imprinting in human pathology.