This chapter provides an evolutionary context to comparative research on monotremes and marsupials. It explains the evolutionary origins of the three major living mammalian clades from within the ancient amniote lineage of synapsids, summarises their most obvious biological differences, and briefly outlines the difference between the terms "Monotremata, Marsupialia and Placentalia" vs. "Prototheria, Metatheria and Eutheria". The living monotreme and marsupial families are introduced via short characterisations of their general biology and evolution. An up-to-date family-level phylogeny is provided for marsupials, together with a summary of our past and current understanding of their phylogenetic relationships. The known fossil record and biogeography of both radiations is summarised; particular attention is given to a recent paradigm shift on monotreme evolution, with the latest research suggesting that monotremes are part of an ancient, Gondwanan radiation of mammals that independently evolved a tribosphenic dentition. The unusual biogeographical history of marsupials and their extinct relatives, including a probable origin in the northern continents and later distribution across South America, Antarctica, and Australia, is also discussed.