Invasive aspergillosis presents a formidable problem for both diagnosis and therapy. Therefore, prevention is a very important strategy in controlling this disease. Preventing invasive aspergillosis demands a clear understanding of the environmental sources of Aspergillus spp. and how this mould is transmitted to patients. Insight into the sources of exposure, mechanisms of transmission, and host susceptibility to infection are vital to appropriately direct preventive strategies to those settings where the risk of infection is the highest and consequently the impact of prevention the greatest.