Infective pneumonia occurring in immunocompromised patients is characterized by its multiple causes which create diagnostic problems and by the need for a prompt treatment. The principal criteria pointing to a specific organism are the causes of immunodeficiency (now usually due to treatment of blood diseases or cancers, organ transplantation and AIDS) and the radiological features of the pneumonia. Confirming the diagnosis frequently requires features of the pneumonia. Confirming the diagnosis frequently requires invasive explorations which in any case are limited by the patient's fragility. Consequently, empirical therapeutic measures are initiated, at least initially, taking into account all likely assumptions. The severity and frequency of these lung infections make it desirable to develop preventive measures applying to the patient himself (antibiotic prophylaxis) or to his environment.