Adult haematopoiesis depends on rare multipotent hematopoietic stem cells (HSCs) that self-renew and give rise to progenitor cells, which differentiate to all blood lineages. The strict regulation of the fine balance between self-renewal and differentiation is essential for normal haematopoiesis and suppression of leukaemia development. HSCs and progenitor cells are commonly assumed to reside within the hypoxic BM microenvironment, however, there is no direct evidence supporting this notion. Nevertheless, HSCs and progenitors do exhibit a hypoxic profile and strongly express Hif-1α. Although hypoxia signalling pathways are thought to play important roles in adult HSC maintenance and leukaemogenesis, the precise function of Hif-dependent signalling in HSCs remains to be uncovered. Here we discuss recent gain-of-function and loss-of-function studies that shed light on the complex roles of hypoxia-signalling pathways in HSCs and their niches in normal and malignant haematopoiesis. Importantly, we comment on the current and often contrasting interpretations of the role of Hif-dependent signalling in stem cell functions. Stem Cells 2014.