Genomic imprinting is a form of non-Mendelian inheritance. It refers to a parental-specific difference in the behaviour of homologous chromosomes or loci. Genes are expressed from two alleles; one maternal and one paternal. But in minor cases, genomic imprinting causes the differential expression of a gene according to its maternal or paternal origin. In this paper, we will discuss examples and the possible molecular mechanisms of imprinting.