Collapse of a lobe of a lung is an important indicator of a range of conditions, including malignancy. Clinical symptoms and signs may suggest a diagnosis of lobar collapse; however, it is often diagnosed, and always needs to be confirmed, with radiological examination. The radiological signs may be subtle, difficult to interpret and sometimes confusing to both clinicians and radiologists. Although multidetector CT (MDCT) is now widely in use for confirming and diagnosing lobar collapse, the plain chest radiograph is usually the first imaging modality performed and so recognition on the plain film remains of vital importance. The basics of chest radiograph interpretation are reviewed, concentrating on the concepts of radiographic density and the silhouette sign. MDCT images are used to demonstrate the general radiological signs of collapse, and the signs of collapse that are specific to the different lobes of the lung are reviewed.