Lobar collapse is important to diagnose when present, as it may indicate the presence of a central bronchial obstructing lesion. In some instances, its appearance is rather classical and easily diagnosed. In other instances, its appearance can challenge even the most experienced radiologist. In yet other instances, it may be simulated by other disease processes. This material represents a discussion of some basic concepts and signs the author considers important in determining whether or not lobar collapse is present.