Genomic imprinting is responsible for monoallelic gene expression that depends on the sex of the parent from which the alleles (one active, one silent) were inherited. X-chromosome inactivation is also a form of monoallelic gene expression. One of the two X chromosomes is transcriptionally silenced in the somatic cells of females, effectively equalising gene dosage with males who have only one X chromosome that is not complemented by a gene poor Y chromosome. X chromosome inactivation is random in eutherian mammals, but imprinted in marsupials, and in the extraembryonic membranes of some placentals. Imprinting and X inactivation have been studied in great detail in placental mammals (particularly humans and mice), and appear to occur also in marsupial mammals. However, both phenomena appear to have evolved specifically in mammals, since there is no evidence of imprinting or X inactivation in non-mammalian vertebrates, which do not show parent of origin effects and possess different sex chromosomes and dosage compensation mechanisms to mammals.¶ In order to understand how imprinting and X inactivation evolved, I have focused on the mammals most distantly related to human and mouse. I compared the sequence, location and expression of genes from major imprinted domains, and genes that regulate genomic imprinting and X-chromosome inactivation in the three extant mammalian groups and other vertebrates. Specifically, I studied the evolution of an autosomal region that is imprinted in humans and mouse, the evolution of the X-linked region thought to control X inactivation, and the evolution of the genes thought to establish and control differential expression of various imprinted loci. This thesis is presented as a collection of research papers that examines each of these topics, and a review and discussion that synthesizes my findings.¶ ...