In the broadest sense, the term immunocompromised individual means a person with any impairment of infection resistance, whether it is deficiency or impairment of innate resistance (by another name impairment of non-specific resistance), or impairment of acquired resistance (impairment of specific immunity). Pneumonia in immunocompromised patients is characterized by a different microbiological spectrum. Apart from common pathogens there are also opportunistic agents involved in this group of patients. Management of pneumonia in immunocompromised patients is often decisive for their future fate. Very rare are innate immune deficiencies. Acquired immune deficiencies are far more common. They are often combined with disorders of non-specific resistance. The most frequent cause of acquired immune deficiency is another illness or a medical treatment. The diagnosing and treatment of pneumonias in immunocompromised patients are usually highly specialized and they unquestionably require a collaboration with microbiologists who are concerned with these problems.