A classification of modulated structures is proposed and electron microscopic studies illustrating the different classes of structures are surveyed. We shall discuss successively the alloys Cu 3Sn, Ni 3MO, Au 4 Zn and the non‐metallic compounds YSeF, barium ferrite, Tas3, TaS2, Ni 3±xTe2 and Cu 3±xTe2.