For many decades, myeloproliferative disorders (MPD) were largely neglected orphan diseases. The conceptual work of William Dameshek in 1951 provided the basis for understanding MPD as a continuum of related syndromes, possibly with a common pathogenetic cause. Recognition of the clonal origin of peripheral blood cells in MPD in 1976 and the ability to grow erythroid colonies in vitro in the absence of added growth factors in 1974 initiated the search for genetic alterations that might be responsible for myeloproliferation. Mutations in the genes for the erythropoietin receptor, thrombopoietin and the von Hippel-Lindau protein were found to cause familial syndromes resembling MPD, but despite their phenotypic similarities, none of these mutations were later found in patients with the sporadic form of MPD. The discovery of activating mutations in the Janus kinase 2 (JAK2) in most patients with MPD has fully transformed and energized the MPD field. Sensitive assays for detecting the JAK2-V617F mutation have become an essential part of the diagnostic work-up, and JAK2 now constitutes a prime target for developing specific inhibitors for the treatment of patients with MPD. Despite this progress, many questions remain unsolved, including how a single JAK2 mutation causes three different MPD phenotypes, what other genes might be involved in the pathogenesis, and what are the factors determining the progression to acute leukemia.