A solitary pulmonary nodule (SPN) has been categorised as a single spherical lesion of less than 3 cm in diameter that is surrounded on all sides by normal lung parenchyma. Nodules, when located in the periphery of the lung, may present a difficulty in diagnosis owing to their relative inaccessibility. This chapter discusses the clinical implications of the descriptors used in defining solid, partially-solid and ground-glass solitary pulmonary nodules, examines the background risk factors for a potential neoplastic aetiology, and discusses the morphology of the nodule in this context. It also describes the aetiological possibilities in some detail, with attention to the distinguishing characteristics of each condition. The chapter also includes a discussion on multiple pulmonary nodules (MPNs), examining their causes and their special features in depth. Strategies involved in the imaging and tissue sampling of pulmonary nodules are also outlined.