In Chapter 3, we introduced a line of research initiated by von Neumann and Penrose around 1960. The ultimate goal of their work was artificial self-reproducing systems. In Chapter 5, we explained our mechanical unit called Fractum, whose goal was self-assembly and self-repair. In fact, the Fractum module was one of the first systems proposed by various researchers after microprocessors became commercially available. In this chapter, in order to help the reader get an overall picture of the research on so-called modular robots, we make a classification of modular robots, identify typical problems in their design, and then give an overview of some representative systems that have been developed.