Polymyalgia rheumatica (PMR) is a disease that rheumatologists feel confident in recognising in their daily practice. In the classic form, its diagnosis is straightforward and standard corticosteroidal treatment usually yields excellent results, at least in the first months. In contrast with the feeling that PMR is a disease that can be readily treated, the aetiology and pathogenesis are still obscure. Unlike several other forms of rheumatic disease, we do not know what the target of inflammation is in PMR. The synovial membrane is the most probable candidate as synovitis has been demonstrated by sonographic and immunohistochemical methods. In addition, the frequent occurrence of peripheral arthritis in patients with PMR further supports this possibility. Synovitis is mild and patchy in extent, which does not explain the amount of inflammation disclosed by symptoms and laboratory tests. The skeletal muscle was considered the main site of inflammation by Barber who coined the name ‘polymyalgia rheumatica’. Although this theory has been discarded for a long time, recent descriptions of the association between PMR and mitochondrial myopathy and of corticosteroid responsive electromyographic abnormalities in patients with PMR have reawakened interest in the role of skeletal muscles. Vasculitis is another attractive possibility because it could comprise PMR and giant cell arteritis (GCA) under a common pathogenesis. The evidence for active vasculitis in biopsy specimens from patients with PMR, however, is scanty. There are also new data that point to bursae as the principal site of inflammation. The large surface of synovial tissue in bursae around glenohumeral joints could explain the amount of inflammation seen in PMR. The main areas of research in PMR aetiopathogenesis are immunology, genetics, and the environment.