Pulmonary hypertension is a significant cause of morbidity and mortality. Unfortunately, non-specific presentation and lack of awareness of the disease frequently lead to significant delay in diagnosis, often with the onset of right heart failure, when prognosis is poor and therapy is of limited effectiveness. The classification of pulmonary hypertension is a clinical one grouping diseases into categories with similar patho-physiological mechanism and therapeutic options. Pulmonary biopsy can provide a definitive diagnosis but is hazardous in patients with pulmonary hypertension. Imaging has emerged as an invaluable tool in differentiating the aetiology, assessing disease severity and directing further management. One of the most important roles of imaging is to differentiate diseases resulting from obstruction of the large pulmonary arteries from those secondary to diffuse small vessel disease, as these have very different prognosis and are also treated differently. Small vessel diseases causing pulmonary arterial hypertension most commonly result from diffuse remodelling of the pulmonary arterioles. There are multiple causes of arteriolar remodelling which share similar histopathological, clinical and imaging features. In a subgroup of small vessel diseases causing pulmonary hypertension the predominant site of increased vascular resistance is at the level of the capillaries or venules. Correct diagnosis of pulmonary veno-occlusive disease and pulmonary capillary haemangiomatosis is essential since poor prognosis and inadvertent administration of vasodilators (conventional therapy for arteriolar predominant disease) can result in fatal pulmonary oedema. Multimodality imaging plays an important role in suggesting a diagnosis, guiding further investigation and directing treatment.