Chest radiographs and CT are frequently used in the diagnosis and monitoring of pulmonary infections. The presence and type of immunocompromise influence which infections a patient may be at risk of contracting. Imaging features often overlap among different infections, but some imaging features may help narrow the differential diagnosis. For example, tree-in-bud nodules suggest mycobacterial, atypical bacterial, or viral pneumonias; diffuse ground glass suggests viral or Pneumocystis pneumonia; and nodules with ground-glass halos suggest invasive fungal pneumonia in neutropenic patients. Septic emboli present with multiple cavitary nodules, and radiologists should search for a source, such as a valvular vegetation, when they encounter such patients. Radiologists should also be aware of entities which can simulate infection on imaging, including organizing pneumonia and malignancies.