The water deer, Hydropotes inermis (Cervidae, Mammalia), is a small, solitary cervid. It is native to China and Korea, but some feral populations also live in Europe. In contrast to other deer species, where males are characterized by antlers and small/no upper canines, H. inermis lacks antlers, but grows long upper canines. For this phenotype and particularities of its biology, the species holds considerable potential not only for our understanding of cervid biology, but also for important questions about basic developmental and regenerative biology. However, H. inermis populations are decreasing, and many of the pressing scientific questions motivated by this peculiar species are still open. Here, we review the most different aspects of the species' biology and discuss scientific publications ranging from the year of the species' first description in 1870 until 2015. We briefly sketch its state of conservation, and we discuss the current understanding of its phylogeny. Lastly, the present overview identifies areas that deserve future research available.