Starting from the lectures EP3 (Optics and Quantum Phenomena) and EP4 (Atomic and Molecular Physics) the lecture discusses light-matter interaction in different systems as well as the quantum nature of light. Emphasis is put onto the laser. Starting from the theory of optical resonators and Gaussian beams we review the generation of laser light on a microscopic level (Maxwell-Bloch equations) and examine its principal characteristics. Various applications of laser light in quantum optics, laser spectroscopy, laser cooling and trapping of atoms and in non-linear optics are investigated. In addition we review various quantum optical phenomena like photon statistics, photon bunching/anti-bunching, multi-photon interferences, intensity interferometers and resonance fluorescence.