Resonant inelastic soft x-ray scattering huge potential is quickly becoming reality. RIXS is element and site selective, like x-ray absorption spectroscopy. It is momentum resolved, like x-ray diffraction. And it probes several kinds of excitations at a time, from charge transfer and electron-hole pairs, to orbital (dd or ff) excitations, to spin waves and lattice modes, unlike any other energy loss spectroscopy. Moreover, the elastic component of the spectra carries information on commensurate and incommensurate orders, such as charge density waves (CDW) and orbital order. The ERIXS endstation at the ID32 beam line of the ESRF is the founder of a new generation of RIXS instruments capable of exploiting all the strongpoints of this technique, thanks to the very high resolving power (30,000 at 1 keV), the diffractometer-like manipulator and the full control of photon polarization provided by the combination of the APPLE II source and the polarimeter on the analyzer. Similar facilities are starting operations at DLS, NSLS II, MAX IV, TPS, to the greatest benefit of beamtime availability worldwide
I will review some of the results obtained in the first years of operations of ID32, with a special focus on cuprate superconductors studied at the Cu L3 edge. High resolution RIXS has been used to determine the relation between crystal structure and the extent of hopping integrals in parent compounds, revealing why apical oxygens are detrimental to superconductivity . Ultra-high resolution RIXS has provided a direct measurement of the momentum-dependent electron phonon coupling in undoped and superconducting samples, and has revealed new collective modes related to charge density waves (CDW) in underdoped Bi2212 . Polarization analysis has definitively demonstrated the spin-flip character of the mid-IR spectral region in superconducting compounds. And the quasi-elastic part of RIXS spectra has brought new evidence of the universality of charge ordering phenomena in cuprates, including striped cuprates  and single layer Bi2201; in particular, by RIXS the charge order can be observed and carefully studied also outside the pseudogap region of the phase diagram, with significant progress in the understanding of its role in high Tc superconductors.
Finally, I will discuss the potential of RIXS at XFEL, with special reference to the hRIXS project that is expected to become operational at SCS in the current year.
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