Recent advancements in single-cell technologies have reshaped our understanding of pulmonary vascular biology, particularly the diverse roles of endothelial cells in chronic lung diseases associated with pulmonary hypertension. This chapter reviews the emerging concept of endothelial heterogeneity in the lung, highlighting how anatomical location and functional specialization define distinct subpopulations, including aerocytes and general capillary endothelial cells. We discuss the identification of endothelial progenitor and stem-like cells and their contributions to vascular remodeling and disease progression. Special attention is given to idiopathic pulmonary fibrosis and chronic obstructive pulmonary disease, illustrating potential mechanisms of endothelial dysfunction and the contribution of processes, including senescence, apoptosis, and endothelial-to-mesenchymal transition, to disease-specific vascular pathology. This chapter underscores the critical role of endothelial cells in lung homeostasis and pathology, emphasizing that targeting endothelial subtypes may open new therapeutic avenues for pulmonary hypertension secondary to chronic lung disease.