The melilite group is one of the feldspathoid minerals and important rock-forming minerals. Crystal structure of melilite group (tetragonal system with the space group P4^^^-2_1m) consists of 8-coordinated W site, and structurally independent two tetrahedral T1 and T2 sites. Main end-members of melilite are akermanite (Ca_2MgSi_2O_7), ferro-akermanite (Ca_2Fe^<2+>Si_2O_7), gehlenite (Ca_2Al^<3+>_2SiO_7), soda melilite (NaCaAlSi_2O_7), ferri-gehlenite (Ca_2Fe^<3+>_2SiO_7), ferri-aluminum gehlenite (Ca_2Fe^<3+>Al^<3+>SiO_7), hardystonite (Ca_2ZnSi_2O_7), gugianite (Ca_2BeSi_2O_7), and okayamalite (Ca_2BSi_2O_7) components. Moreover, Sr and Ba substitute for Ca, and, thus, chemical compositions of melilite group vary in a wide range. Essential subjects for the melilite group are 1) cation distributions in the T1 and T2 sites and their controlling factors, and 2) the effect of ionic substitutions at the W, T1 and T2 sites on the crystal structure, especially on the incommensurate structure. These subjects have been studied systematically using X-ray diffraction, electron diffraction and Mossbauer spectroscopic methods. In this paper, the authors review the studies on 1) the controlling factor for distribution of cation in the T1 and T2 sites, and 2) the relationship between ionic distributions and incommensurate structure.