The identification of mutations in the bone morphogenetic protein (BMP) type II receptor in the majority of cases of familial pulmonary arterial hypertension (PAH) has provided a focus for researchers studying the complex pathobiology of this condition. Mutations are also found in a proportion of idiopathic PAH cases and it is now emerging that dysfunctional BMP signaling plays a role in other more common forms of PAH, even in the absence of mutations in the gene. Study of the role of BMP signaling in endothelial, smooth muscle cell, progenitor cell and inflammatory cell biology may reveal novel pathways lending themselves to therapeutic intervention in PAH. This chapter summarizes the present status of our understanding of the role of BMPR-II mutations in PAH and indicates future directions for research.