Vasculitis encompasses a heterogeneous group of autoimmune disorders characterised by inflammation and necrosis of blood vessel walls. The resulting vessel wall damage causes luminal narrowing, thrombosis, aneurysm formation, and end-organ ischaemia. Vasculitis is classified by the size of predominantly affected vessels (large, medium, or small), the presence of ANCA antibodies, and specific clinicopathological features. It may be primary (idiopathic autoimmune) or secondary to infection, malignancy, drugs, or other connective tissue diseases. Accurate classification is essential as treatment strategies differ significantly between subtypes.