3D seismic data provide images of geological features which are approximately circular in plan view but whose shape and origin may not be possible to constrain using 2D seismic data. As 3D seismic data become more commonly employed in hydrocarbon exploration, the number of demonstrably "circular" structures will increase. At least ten different geological processes can result in seismically resolvable "circular" structures in sedimentary basins. These include salt/shale diapirs, salt withdrawal basins, polygonal fault blocks, dissolution collapse hollows, breccia pipes, calderas, gas pockmarks, bioherms, sand volcanoes, pull-aparts, impact craters and tectonic folds. Geometrical and geological criteria for each are summarized to facilitate identification of such features should they be encountered in the course of a 3D seismic interpretation. Certain types of geological feature have distinctive properties, others are less straightforward to recognize on the basis of individual criteria.