The importance of the Turkana Basin (northern Kenya) and lower Omo Valley (southern Ethiopia) in our understanding of the environments in which early hominins lived, and on how climate changes may have influenced the emergence and disappearance of species, is undeniable (e.g., Bobe and Leakey, 2009; Levin et al., 2011). The majority of the Plio-Pleistocene sediments belong to the Omo Group, which consists of the Shungura, Usno, Nkalabong, and Mursi formations in Ethiopia and of the Nachukui and Koobi Fora formations in Kenya (e.g., Brown and McDougall, 2011). The Shungura, Nachukui, and Koobi Fora formations are well-studied (e.g., Howell and Coppens, 1976; de Heinzelin, 1983a, 1983b; Harris et al., 1988a, 1988b; Koobi Fora Research Project Volumes). The Mursi Formation of the lower Omo Valley, with a date of more than 4 million years, is among the oldest formations of the Omo Group in Ethiopia. However, it has been poorly documented despite its importance in informing us on the mid-Pliocene, a period in which the genus Australopithecus arose, but that is presently inadequately known. This chapter will present the formation with the history of the research in the area, including renewed fieldwork. It will be followed by a description of the geology and taphonomy and, finally, a preliminary analysis of the fauna found to date.