Resonant inelastic X-ray scattering (RIXS) is a powerful technique that combines spectroscopy and inelastic scattering to probe the electronic structure of materials. RIXS is based on the interaction of X-rays with matter in which the dependence on energy, momentum and polarization is introduced. The RIXS spectra can be approximated as a combination of X-ray absorption and X-ray emission. A 2D RIXS plane can be measured as a function of excitation and emission energies. Using RIXS, collective excitations -such as magnons, phonons, plasmons and orbitons -can be probed in quantum materials, for example, cuprates, nickelates and iridates, with complex low-energy physics and exotic phenomena in energy and momentum space. In addition, RIXS with hard X-rays enables detailed experiments under operando conditions. Spectral broadening owing to short core hole lifetime can be reduced to produce X-ray absorption spectra with high resolution. This Primer gives an overview of RIXS experimentation, data analysis and applications, finishing with a look to the future, where new experimental stations at X-ray free electron lasers promise to revolutionize the understanding of femtosecond processes and non-linear interaction of X-ray with matter