The basic principles of the formation of a stereocomplex between isotactic and syndiotactic poly(methyl methacrylate)s (i-PMMA, s-PMMA) are surveyed. The influences of polymer stereoregularity, i-PMMA/s-PMMA ratio in the mixture, time, concentration of polymers, temperature and thermal history on the stereocomplex formation are discussed. The proposed model of PMMA stereocomplex and the course of the stereocomplex formation are described. Attention is also paid to the self-aggregates of s-PMMA and i-PMMA.