Crystals of double phosphates ABP2O7 belong to the family of complex phosphates of the alkaline (A) and polyvalent (B) metals. Some of the double phosphates e.g. NaTiP2O7 is a perspective material for development of new crystallophosphors, since they have stable, chemically and thermally inert crystal lattice. Moreover, new luminophors can be designed by on the base of solid solutions of the diphosphate matrixes. We present results of complex theoretical and experimental investigations of double phosphate crystals ABP2O7 (A = K, Na, Cs; B = Al, In, Ti). The investigated compounds were obtained by the “melting solution” method in 650 - 1050 C temperature region at ambient atmospheric conditions. The electronic structures of perfect NaTiP2O7, KAlP2O7, LiInP2O7 and NaAlP2O7 crystals are calculated by full potential Linear Augmented Plane Wave (FLAPW) using WIEN2k program package [1]. The structures of the one-electron bands, the energy dependencies of the components of dielectric tensor, reflection and absorption spectra are obtained. Photoluminescence (PL) and PL excitation spectra of ABP2O7 (A = K, Na, Cs; B = Al, In, Ti) crystals were obtained at 4.2, 77 and 300 K. Luminescence was excited by radiation from the N2-laser, the Ar-laser, and the powerful xenon lamp. The luminescence was registered by various spectrometers, which cover wide spectral region. Reflection, excitation and luminescence spectra were investigated using synchrotron radiation in the energy region 3.5 – 20 eV. Experiments with synchrotron radiation were carried out on SUPERLUMI station at HASYLAB (DESY), Hamburg, Germany. Experimental data are analyzed along with results of theoretical calculations. General inferences concerning participation of A, B cations, and P2O7 phosphate groups in formation of the luminescence spectra and other spectroscopic properties of all investigated phosphate compounds are made. The role of defects in formation of the spectroscopic properties of ABP2O7 crystals is discussed. The origin of the ABP2O7 luminescence is clarified.