Beckwith-Wiedemann syndrome (BWS) is an overgrowth disorder characterized by developmental anomalies, tissue and organ hyperplasia, and an increased risk of embryonal tumors. BWS is a multigenic disorder with dysregulation of the expression of imprinted genes involved in growth and cell cycle control in the 11p15 chromosomal region. The molecular genetics appear to involve either mutations of imprinted genes or chromosomal alterations of a putative imprinting center that controls gene expression across a large chromosomal domain. As the molecular genetics underlying BWS are unraveled, our knowledge of this imprinting disorder and its relationship to neoplasia and developmental anomalies will be expanded. Currently available diagnostic testing and follow-up protocols will evolve as our understanding of the molecular basis of this disease progresses.