Primary systemic vasculitis is a group of heterogeneous disorders, characterized by inflammation of blood vessels causing end organ damage from ischemia, aneurysm formation or dissection. Delay in the early diagnosis owing to non-specific symptoms, lack of definitive serological tests, limited availability of biopsy and standard imaging tests pose significant challenge in the management of these diseases. 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning is being increasingly used in the management of systemic vasculitis, especially the large vessel vasculitides: giant cell arteritis (GCA) and Takayasu arteritis (TAK). FDG-PET involves detection of positron rays emitted by FDG, a fluorinated glucose analogue which is avidly taken up by metabolically active inflammatory cells in the walls of involved blood vessels. 18F-FDG-PET scan, especially when combined with computed tomography or magnetic resonance imaging (MRI) can give information about active inflammation as well as structural damage associated with vasculitis. In patients with GCA, FDG-PET has acceptable sensitivity and specificity for the early diagnosis in non-cranial GCA, cranial GCA with negative biopsy, assessment of immediate response to treatment, predicting prognosis, but has limited value in serial follow-up and prediction of relapses. In TAK, FDG-PET can be useful for early diagnosis and probably for serial assessment of disease activity. FDG-PET has a limited role in medium and small vessel vasculitis.