Studies of the long-period ordered alloys are surveyed. Alloys that are in most cases f.c.c. in a disordered state often transform into that ordered state composed of regular antiphase domains which is represented by CuAuII but realized in many A3B type alloys such as Ag3Mg, Cu3 Pt II, Pd3Mn, Au3Cd, Au3Zn(H), Au3+Zn, Au3Mn, Cu3Pd(α″) etc. Many structural studies have been carried out mainly by X-ray and e-lectron diffraction and in some cases by electron microscopy. In the present paper features of these long-period ordered alloys such as change of the period with composition and temperature, non-integrality of the period, one-dimensional and two-dimensional domains, lattice modulation accompanying the long period and contrasts of electron micrographs of thin films are described. Origin of the long period has been studied theoretically, and some of studies based on a current theory are described.