Atranes are comprised of two bridgehead atoms bridged by three three-atom moieties. When the bridgehead atoms interact, a [3.3.3.0]tricyclic system is produced; when they do not, a [3.3.3]bicyclic structure is evident. Quasi-tricyclic structures are also known in which the bridgehead-bridgehead bond length lies between the sum of the van der Waals radii and a normal transannular bond. Because of this and additional factors, atranes give rise to interesting new and useful chemistry as well as novel molecular architectures. In this review the chemical reactivities and structural properties of main group atrane systems are surveyed.