Aspergillosis is an infectious, non-contagious fungal disease caused by species in the ubiquitous opportunistic saprophytic genus Aspergillus, in particular Aspergillus fumigatus. This mycosis was described many years ago, but continues to be a major cause of mortality in captive birds and, less frequently, in free-living birds. Although aspergillosis is predominantly a disease of the respiratory tract, all organs can be involved, leading to a variety of manifestations ranging from acute to chronic infections. It is believed that impaired immunity and the inhalation of a considerable amount of spores are important causative factors. The pathogenesis, early diagnostic methods and antifungal treatment schedules need to be further studied in order to control this disease. The aim of the present review is to present the current knowledge on aspergillosis with the main emphasis on A. fumigatus infections in captive and free-living birds rather than domestic poultry. The review covers aetiology, epidemiology, pathogenesis, clinical signs and lesions, diagnosis, treatment and prevention.