In mammals, the most frequently observed hybrid dysgenesis effects are growth disturbances and male sterility. Profound defects in placental development have been described and our work on hybrids in genus Mus has demonstrated putative hybrid dysgenesis effects that lead to defects in lipid homeostasis and maternal behavior. Interestingly, mammalian interspecies hybrids exhibit strong parent-of-origin effects in that offspring of reciprocal matings, even though genetically identical, frequently exhibit reciprocal phenotypes. Recent studies have provided strong link between epigenetic regulation and growth, behavior and placental development. Widespread disruption of genomic imprinting has been described in hybrids between closely related species of the genus Peromyscus. The studies presented in this thesis aim to investigate the effects of disrupted epigenetics states on altered growth, female infanticide and placental dysplasia observed in Mus hybrids. We showed that loss-of-imprinting (LOI) of a paternally expressed gene, Peg1, was correlated with increased body weight of F1 hybrids. Furthermore, we investigated whether LOI of Peg1 in F1 females would interfere with maternal behavior. A subset of F1 females indeed exhibited highly abnormal maternal behavior in that they rapidly attacked and killed the pups. By microarray hybridization, a large number of differentially expressed genes in the infanticidal females as compared to normally behaving females were identified. In addtion to Peg1 LOI, we studied allelic expression of numerous imprinted genes in adult Mus interspecies hybrids. In contrast to the study from Peromyscus, patterns of LOI were not consistent with a direct influence of altered expression levels of imprinted genes on growth. Finally, we investigated the allelic interaction between an X-linked locus and a paternally expressed gene, Peg3, in placental defects in Mus hybrids. This study further strengthened the notion that divergent genetic and epigenetic mechanisms may be involved in hybrid dysgenesis in diverse groups of mammals.