To understand the conditions under which mutation, recombination and selection can lead to complex adaptations is of importance for evolutionary biology as well as its applications in computer science (evolutionary algorithms). The central idea uniting these two fields is the insight that the genetic representation of the phenotype determines its evolvability (the so-called representation problem). A recurrent theme in the biological literature is the concept of modularity, the fact that higher organisms are composed of semi-autonomous units (gene nets, Bonner (1988); dissociability, Needham (1933) and Gould (1977); individuality, Wagner (1989b,c); selfmaintaining organizations, Fontana and Buss (1994); developmental modules, Raff, 1995). However, even if the fact and the importance of modularity seems to be widely appreciated, there is little understanding of what selective forces have generated genetic and developmental modularity.