SIG3 is the Special Interest Group on Aperiodic Crystals established in August 1998 by European Crystallographic Association (ECA).

According to IUCr [Acta Cryst. A48(1992), p. 928] by “crystal” we mean any solid having an essentially discrete diffraction diagram, and by “aperiodic crystal” we mean any crystal in which three dimensional lattice periodicity can be considered to be absent. The mission of both SIG3 and CAC is promotion of experimental and theoretical research on aperiodic crystals, including quasicrystals, modulated crystals, composite crystals, magnetic systems, and polytypes.