Pulmonary hypertension is a life-threatening disease with no known cure. Here we provide a concise yet comprehensive review of the current knowledge about the pathophysiology of pulmonary hypertension (PH). The underlying signaling mechanisms involved in pulmonary vascular remodeling and the exaggerated vascular contractility, two characteristic features of pulmonary hypertension, are discussed in depth. The roles of inflammation, immunity, and right ventricular function in the pathobiology of pulmonary hypertension are discussed. The epidemiology of the five groups of pulmonary hypertension (World Health Organization classification; Nice, 2013) is also briefly described. A clear understanding of our current knowledge about the pathogenesis of PH is essential for further exploration of the underlying mechanisms involved in this disease and for the development of new therapeutic modalities. This book should be of interest to researchers and graduate students, both in basic research and in clinical settings, in the fields of pulmonary vascular biology and pulmonary hypertension.