The myeloproliferative disorders (MPDs) are a group of hematologic diseases with significant overlap in both clinical phenotype and genetic etiology. While most often caused by acquired somatic mutations in hematopoietic stem cells, the presence of familial clustering in MPD cases suggests that inheritance is an important factor in the etiology of this disease. Though far less common than sporadic disease, inherited MPDs can be clinically indistinguishable from sporadic disease. Recently, germline mutations in Janus kinase 2 (JAK2) and MPL, two genes frequently mutated in sporadic MPD, have been shown to cause inherited thrombocytosis. Study of the function of these mutant proteins has led to a new understanding of the biological mechanisms that produce myeloproliferative disease. In this review, we summarize the data regarding inherited mutations that cause or predispose to MPDs, with a focus on the biological effects of mutant proteins. We propose that defining inherited MPDs in this manner has the potential to simplify diagnosis in a group of disorders that can be difficult to differentiate clinically.