Inadequate oxygen availability or hypoxia induces a complex and still incompletely understood set of adaptations that influence cellular survival and function. Many of these adaptations are directly controlled by a master transcription factor, hypoxia inducible factor-alpha (HIF-α). In response to hypoxia, HIF-α levels increase and directly induce the transcription of > 100 genes, influencing functions ranging from metabolism, survival, proliferation, migration, to angiogenesis, among others. Recently, it has been demonstrated that a specific set of microRNA molecules are upregulated by hypoxia, which we denote here as "hypoxamirs." In particular, the HIF-responsive hypoxamir microRNA-210 (miR-210) is a unique microRNA that is evolutionarily conserved and ubiquitously expressed in hypoxic cell and tissue types. A number of direct targets of miR-210 have been identified by in silico, transcriptional, and biochemical methods, a subset of which have been extensively validated. As a result, miR-210 has been mechanistically linked to the control of a wide range of cellular responses known to influence normal developmental physiology as well as a number of hypoxia-dependent disease states, including tissue ischemia, inflammation, and tumorigenesis. Thus, reflecting the pleiotropic actions of HIF-α, miR-210 appears to function as a "master microRNA" relevant for the control of diverse functions in the hypoxic state.