Genomic imprinting is an epigenetic mechanism that silences one parental allele of a small subset of genes. Many imprinted genes exhibit this property only in extraembryonic tissues-placenta and yolk sac. This has led to the idea that imprinting in mammals coevolved with some aspect of placentation. Nevertheless, many studies of imprinting have ignored the extraembryonic tissues, the yolk sac and its precursor, the primitive endoderm, in particular. The primitive endoderm is involved in very early signaling events during a critical stage in development, gastrulation, during which body plan axes and head process neuroectoderm are established. Improper signaling from primitive endoderm as a result of abnormal expression of imprinted genes has the capacity to effect long-term defects in embryonic/fetal tissues that might hitherto have been overlooked. We discuss these gaps in the knowledge, propose a mechanism for genomic imprinting based on current data, and suggest a line of investigation that will expand our understanding of this unique regulatory mechanism and its impact on development.