Imprinting is a crucial phenomenon in the epigenetic regulation of flowering plants and mammals, where the preferential expression of alleles depends on their parental origin. Disruption of genomic imprinting usually causes abortion of early zygotic development in both plants and mammals. Genomic imprinting arises from extensive chromatin remodeling within the epigenome, typically involving clustered imprinting genes flanked by transposable elements. Consequently, the differential expression of alleles is closely linked to DNA methylation and histone modifications, mainly histone 3 lysine 27 tri-methylation (H3K27me3). In plants, imprinting predominantly occurs in the endosperm, a triploid placenta-like tissue that envelops and nurtures the embryo during seed development. Therefore, imprinted genes are involved in not only seed development but also the establishment of seed dormancy, an important adaptive trait for plant evolution. This chapter provides a comprehensive review of established knowledge and recent advancements in understanding the regulation of genomic imprinting, the key factors influencing these processes, and the biological functions associated with genomic imprinting.