This chapter explores theories of revolution that propose psychological factors as the main driving force for intergroup change. These theories include social identity theory, psychodynamic theory, justice theories, equity theory, and relative deprivation theory. A number of the theories that also give importance to material conditions, such as system justification theory, also give importance to psychological factors through concepts such as false consciousness. The irrationalist perspective of psychodynamic theory is also integral to a number of the other theories discussed in this chapter because of the pervasive assumption that often people are unaware of what is influencing their participation in collective action and displacement of aggression. The most influential contemporary theory is social identity theory, and the key experiment associated with this theory, the minimal group paradigm, is critically discussed. The complex role of relative deprivation and perceived justice is discussed, with reference to fraternal deprivation and equity theory.