A subset of mammalian genes is controlled by genomic imprinting. This process causes a gene to be expressed from only one chromosome homologue depending on whether it originally came from the egg or the sperm. Parental origin-specific gene regulation is controlled by epigenetic modifications to DNA and chromatin. Genomic imprinting is therefore a useful model system to study the epigenetic control of genome function. Here we consider the value of the mouse as an experimental organism to address questions about the role of imprinted genes, about the regulation of mono-allelic gene expression and about the evolution of the imprinting function and mechanism.