A radiological sign can sometimes resemble a particular object or pattern and is often highly suggestive of a group of similar pathologies. Awareness of such similarities can shorten the differential diagnosis list. Many such signs have been described for X-ray and computed tomography (CT) images. In this article, we present the most frequently encountered plain film and CT signs in chest imaging. These signs include for plain films the air bronchogram sign, silhouette sign, deep sulcus sign, Continuous diaphragm sign, air crescent ("meniscus") sign, Golden S sign, cervicothoracic sign, Luftsichel sign, scimitar sign, doughnut sign, Hampton hump sign, Westermark sign, and juxtaphrenic peak sign, and for CT the gloved finger sign, CT halo sign, signet ring sign, comet tail sign, CT angiogram sign, crazy paving pattern, tree-in-bud sign, feeding vessel sign, split pleura sign, and reversed halo sign.