A cyst is a thin walled, fluid containing space lined by epithelium (fluids include gases and liquids) Cysts must be differentiated from other air or liquid containing structures which are associated with a different set of etiological conditions. A differential diagnosis of cysts is constructed based on their location within the lung, their size and morphology and the presence/absence of various abnormalities in the surrounding lung parenchyma (such as ground-glass opacities, nodules or emphysema). Cysts may occasionally be malignant, and cysts that appear potentially malignant need to be identified and promptly investigated.