Pulmonary artery vasoconstriction and vascular remodeling greatly contribute to a sustained elevation of pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) in patients with pulmonary arterial hypertension (PAH). The development of PAH involves a complex and heterogeneous constellation of multiple genetic, molecular, and humoral abnormalities, which interact in a complicated manner, presenting a final manifestation of vascular remodeling in which fibroblasts, smooth muscle and endothelial cells, and platelets all play a role. Vascular remodeling is characterized largely by medial hypertrophy due to enhanced vascular smooth muscle cell proliferation or attenuated apoptosis and to endothelial cell over-proliferation, which can result in lumen obliteration. In addition to other factors, cytoplasmic Ca2+ in particular seems to play a central role as it is involved in both the generation of force through its effects on the contractile machinery, and the initiation and propagation of cell proliferation via its effects on transcription factors, mitogens, and cell cycle components. This review focuses on the role played by cellular factors, circulating factors, and genetic molecular signaling factors that promote a proliferative, antiapoptotic, and vasoconstrictive physiological milieu leading to vascular remodeling.