Over the past ten years, on-demand single photon generation has been realized in numerous physical systems including neutral atoms, ions, molecules, semiconductor quantum dots, impurities and defects in solids, and superconductor circuits. The motivations for generation and detection of single photons are two-fold: basic and applied science. On the one hand, a single photon plays a central role in the experimental foundation of quantum mechanics and measurement theory. On the other hand, an efficient and high-quality single-photon source is needed to implement quantum key distribution, quantum repeaters and photonic quantum information processing.