The intimal lining of all blood vessels is composed of a single continuous layer of squamous epithelial cells which are known as endothelial cells (ECs). The vascular endothelium is a “dynamic” highly specialized tissue with multiple physiological, immunological, and metabolic functions. Currently, the endothelium is viewed as a tissue comprising functionally heterogeneous ECs, depending on the organ with which it is associated, and the vascular bed location. ECs may be affected by numerous noxious agents and compounds, leading to diverse “endotheliopathies” present in a wide spectrum of acute and chronic abnormalities, such as sepsis, acute lung injury (ALI), and pulmonary arterial hypertension. In the human lung, ECs occupy a surface area of approximately 130 m2. The strategic location of the lungs, combined with the tremendous surface area of the pulmonary microvascular endothelium, allows for interaction with the entire circulating blood volume before it enters the systemic circulation. Thus, pulmonary endothelial functional and structural integrity are essential for the maintenance of adequate pulmonary and systemic cardiovascular homeostasis. This chapter focuses on the physiology and pathophysiology of the pulmonary endothelial metabolic functions, on methods that measure pulmonary microvascular metabolic activities in vivo, and on the application of such methods in the clinical setting, especially in patients suffering from pulmonary hypertension.