The role of defects in phase transitions is an important subject, especially in systems of low dimensionality. The two-dimensional system Sn/Ge(1 1 1)- ( 3 × 3 )R 30° is an excellent example. The surface undergoes a gradual ( 3 × 3 ) to (3×3) phase transition upon cooling, and much debate has arisen over the exact physical mechanism responsible for the transition. Concepts such as Fermi surface nesting, charge density waves, Jahn–Teller distortions, metal–insulator fluctuations, and soft phonons have been invoked to explain the experimental findings, and there is now a growing understanding that substitutional Ge defects play an important role. A brief historical overview is presented of the work done on the Sn/Ge(1 1 1) system and of the different reported explanations of the phase transition. An extensive discussion of the phase-transition mechanism, and of the role played by defects, is provided. It is argued that the transition should be classified as displacive-like rather than order–disorder-like, but that defects obscure a clear distinction. The definitive experiments to test this conclusion are suggested.