Materials with strong electronic correlations are in the focus of solid state research because they exhibit interesting novel physical properties such as metal-insulator transitions, colossal magneto-resistance, quantum criticality and high-temperature superconductivity etc. Phenomenon of superconductivity in such systems also has been already used in a many technological applications is still not well understood from both theoretical and experimental perspectives. Angle-resolved photoemission spectroscopy (ARPES) allows studying fingerprints of interactions between electronic quasiparticles in such correlated electron systems. Applied to high-temperature superconductors, this technique provides decisive information about the mechanism of pairing.