Late Miocene, 8 to 6 million years ago (Ma), climatic changes brought about dramatic floral and faunal changes. Cooler and drier climates that prevailed in the Late Miocene led to expansion of grasslands and retreat of forests at a global scale. Palaeogeographic studies suggest a global vegetation change causing an abrupt increase in C4 plant biomass while C3 biomass decreased between 8 and 6 Ma. Subsequent cycles of cooler and drier climatic conditions during the Mid-Pliocene (3.5–3 Ma) and Pleistocene (2.8–2.5; 1.8–1.6; 1.0–0.8 Ma) also caused forests to retreat into isolated refugia which played an important role in events that led speciation and radiation of Muroid (Order Rodentia, Superfamily Muroidea) rodents. Muroid rodents are comprised of 6 families (Placanthomyidae, Spalacidae, Calomyscidae, Nesomyidae, Cricetidae, and Muridae) and make up close to one-third of named mammal species. Family Cricetidae and Muridae are especially speciose (containing ~1600 species altogether) and much of the diversity within these families arose during or after the Late Miocene. My dissertation deals with the systematics and historical biogeography of these fast-evolving groups of rodents with an emphasis on the genera Apodemus and Hybomys (Subfamily Murinae, Family Muridae), and Neodon (Subfamily Arvicolinae, Family Cricetidae). Habitat specialists such as Apodemus that occupy broadleaf forests, and Hybomys that occupy rainforests were likely isolated in forest refugia after the retreat of forests facilitating allopatric speciation. While voles in the subfamily Arvicolinae, that are associated with grasslands, expanded their range when forests retreated and speciated when grasslands retreated. In addition, field work carried out for this project in Nepal included several localities previously not sampled for small mammals. Most of Nepal is poorly surveyed and the first chapter focuses on the history of mammalogical surveys in Nepal and adds new localities for small mammal species, expanding the known range of the Nepalese endemic Himalayan wood mouse (Apodemus gurkha).