X chromosome inactivation (XCI) is a mammalian dosage compensation mechanism that ensures balanced expression of X-linked genes between males and females. Research using rodent models has led to major discoveries regarding XCI mechanisms and dynamics, in addition to the molecular actors involved in this process, including the long noncoding RNA Xist and its protein partners. However, several features of XCI vary significantly across mammalian species, not only between marsupials and placental mammals, but also within the latter. This review discusses the fundamental aspects of XCI from an evolutionary perspective, highlighting both conserved features and species-specific variations across mammalian species.