Pulmonary disease in the immunocompromised host (ICH) remains a major cause of morbidity and mortality. Mortality has remained high at 40% to 50%, 123, 139 especially in patients who develop diffuse infiltrates. 125 The incidence of infections varies with the underlying disease, for example, pulmonary infections in neutropenic and renal transplant patients are less common than in heart or heart-lung transplant patients. The incidence is estimated at 30% to 60% 3, 38 in bone marrow transplant patients, although recent studies have shown incidences much lower, in the range of 2.5% to 5%, 60, 92 in patients who received peripheral blood stem cell transplantation and growth factors. The incidence in liver transplant patients is around 11% to 15%, 133, 140 heart transplants between 14% and 38%, 89, 146 and renal transplants less than 10%. 91 Incidence has remained above 50% in the lung 37, 76 and heart-lung transplant population. 62 Pneumonia in the neutropenic population, on the other hand, is comparatively low and estimated at 0.5% to 10%. 42, 60, 105 In this chapter we address the causes and temporal occurrence of the most common pathogens in the neutropenic, solid organ and bone marrow transplant patient. We focus on the differential diagnosis and raise relevant points regarding each of the more important implicated pathogens. We conclude with a general approach toward diagnosis and empiric therapy.