Pneumonia is a ubiquitous killer in all patient populations but presents a particular risk for morbidity and mortality among immunosuppressed cancer patients. The enhanced susceptibility of cancer patients to pneumonia arises from the aggregate effects of disease- and treatment-related immune dysfunction, disease- and treatment-related anatomic derangements, and healthcare-associated pathogen exposures, with the exact nature of an individual patient’s susceptibility being highly unique. Further, while prompt initiation of adequate antimicrobial therapy is essential to improved pneumonia outcomes, the initial diagnosis of pneumonia is often challenging in this population, as cancer patients with pneumonia frequently present with attenuated clinical signs and symptoms and very often have competing diagnoses that might explain new radiographic infiltrates, fever, or respiratory symptoms. This chapter addresses the mechanisms underlying pneumonia susceptibility in cancer patients, discusses the pathogens most often encountered, and provides an overview of pneumonia preventative, diagnostic, and therapeutic strategies in this vulnerable patient group.