The etiology of giant cell arteritis and polymyalgia rheumatica remains unknown, although the HLA-DR4 group and the pre-existence of a degenerative vascular disease are confirmed risk factors. The incidence may vary between countries, but the North-South gradient should be considered with caution because of potential detection and collection bias. Infectious trigger factors have been looked for both at the epidemiological and biological level: annual, cyclic variations of incidence have been shown in Minnesota, seasonal variations in Scotland, France or Israel. The pre-existence of clinical, mainly respiratory, infection has been suggested in one study, but not confirmed afterwards. Simultaneous occurrence of peaks of GCA/PMR and respiratory infections have been observed in Denmark. Several viruses have been suspected as triggers and assessed by serological testing, PCR or immunostaining on temporal artery biopsies, or both techniques: the hepatitis B virus can be ruled out, as well as Herpes simplex 1 and 2, Herpes varicellae, Epstein-Barr virus and cytomegalovirus. Recent studies focused on parainfluenza virus, Parvovirus B19 and Chlamydia pneumoniae. Immunological studies suggest, at the origin of the inflammatory reaction leading to the typical pathological features of giant cell arteritis, the existence of a triggering antigen of unknown nature activating T-cells in the artery wall.