Aspergillosis has become a major fungal infection in hospitals since the advent of immunosuppressive therapy in the last fifteen years. Aspergilli are saprophytic and ubiquitous fungi and are associated with pulmonary and disseminated infections in immunodepressed patients with a mortality rate of about 85%. Aspergillosis is an air-borne infection, thus intensive care units should be conceived so as to decrease the outer risk of contamination. In the first part of this chapter the conception of such a unit is presented, taking the bone marrow transplantation unit of St-Louis Hospital as an example. A prospective study of the quality of the environment is a complementary and necessary information. In the second part of this chapter, the methods of airborn control of Aspergillus sp. are investigated. According to the literature, commun concepts can be drawn to evaluate nosocomial risks of aspergillosis. Nevertheless in the absence of a defined and accepted methodology, it is not possible up to that date to propose norms and acceptable norms of levels of contamination adapted to the degree of immunodepression of patients.