Abstract: Cavity exciton-polaritons are quasi-particles formed by the strong coupling between excitons and cavity photons in microcavities. They exhibit the dual nature of light and matter, and have significant promising applications in both fundamental physics and future photonic circuits. We review the historical development and important achievements of this field since the first experimental demonstration of cavity polaritons thirty years ago, followed by a survey of future research avenues. A detailed description of the basic concepts of exciton-polaritons and the polariton Bose—Einstein condensation is provided, as well as a brief introduction to the photonic fluid, state control and quantum effects of cavity polaritons.