Chronic thromboembolic pulmonary hypertension (CTEPH) is precapillary pulmonary hypertension, in which chronic obstruction of large and middle branches of pulmonary arteries (PAs) and secondary changes in the lung microcirculatory bed result in a progressive increase in pulmonary vascular resistance and PA pressure with the development of severe right cardiac dysfunction and heart failure. CTEPH is a unique form of pulmonary hypertension since it is potentially curable by surgical treatment. The diagnostic criteria for CTEPH are a mean PA pressure of ≥25 mm Hg, as evidenced by right heart catheterization; a PA wedge pressure of ≤15 mm Hg; a pulmonary vascular resistance of >2 Wood units; the presence of chronic/organized thrombi/emboli in the elastic PAs (pulmonary trunk, lobular, segmental, subsegmental PAs); effective anticoagulant therapy at therapeutic dosages over at least 3 months. Up to now, our country has had no guidelines for the diagnosis and treatment of this rare severe disease that, when appropriately untreated, has an extremely poor prognosis. The main task in the preparation of this document was to generalize and analyze the data of current registries, multicenter randomized clinical trials, national and international guidelines, and consensus documents recently published on this problem in order to optimize a diagnostic process and treatment in this category of patients. Part 1 gives a definition of CTEPH, its place in the clinical classification, epidemiology and prognosis, risk factors, pathogenesis and morphology, diagnostic approaches and determination of operability in patients, and specific features of differential diagnosis.