Polariton-hybridization of light-matter oscillations can emerge from various quasiparticles, such as phonon, plasmon, exciton and magnon. Particularly, exciton polaritons are bosonic quasiparticles with half-light, halfmatter nature, which are originated from strong coupling between excitons and microcavity photons. The half-light nature results in extremely small effective mass, making it feasible to achieve high temperature even room-temperature Bose-Einstein condensation (BEC). Meanwhile, the half-matter nature leads to strong nonlinear interaction, which is missing between photons and can promote the polaritons relaxation to ground state and give rise to low threshold polariton lasing, compared to photonic lasing. The exciton polaritons are of great importance in applications of quantum simulation, topological quantum optics, ultrafast optical switch and low threshold lasers.