The Prader-Willi and Angelman syndromes are now well established as the paradigm of genomic imprinting in human disease. Over the past year, much has been learnt about the mechanisms by which these syndromes arise and molecular diagnostics for the majority of patients are now available. Mouse models for aspects of the syndromes have been established, and the first association between a gene, located in chromosome 15, at 15q11-q13, and a phenotype (albinism) has been proven. Large parts of the critical regions have been cloned and at least six genes identified. Three genes or DNA sequences may be imprinted: two of these demonstrate DNA-methylation imprints and one is functionally imprinted in mouse. While the molecular mechanism of imprinting is not yet understood, it is beginning to yield its secrets to DNA methylation, replication, and chromatin structure studies of the phenomenon.