This review discusses the progress in multi-photon interferometry and its applications in quantum mechanics and quantum information processing. The focus is on the creation and control of photonic entanglement, tests of quantum mechanics, quantum communication protocols, and quantum computation with linear optics. The review emphasizes "few photon" phenomena involving discrete observables. Multi-photon interference reveals strictly non-classical phenomena. It has applications in fundamental tests of quantum mechanics and photonic quantum information processing. The review covers theoretical and experimental progress in this area, including the creation of photonic entanglement, tests of quantum mechanics, quantum communication protocols, and quantum computation with linear optics. The review discusses the basics of interference and quantum entanglement, including classical and quantum interference, quantum entanglement, and interferometry with entangled two- and multi-photon states. It also covers photonic qubits and linear optics, experimental realizations of photonic entanglement, and the falsification of a realistic world view. The review discusses the applications of photonic entanglement in quantum communication, including quantum dense coding, quantum teleportation, entanglement swapping, and quantum memory and quantum repeaters. It also covers photonic quantum computing, including linear-optical two-qubit logic gates, cluster-state quantum computing, and few-photon quantum computing experiments. The review concludes with a discussion of the future of photonic quantum information processing, emphasizing the importance of further research in this area. The review is based on a comprehensive analysis of the literature on multi-photon interferometry and its applications in quantum mechanics and quantum information processing.This review discusses the progress in multi-photon interferometry and its applications in quantum mechanics and quantum information processing. The focus is on the creation and control of photonic entanglement, tests of quantum mechanics, quantum communication protocols, and quantum computation with linear optics. The review emphasizes "few photon" phenomena involving discrete observables. Multi-photon interference reveals strictly non-classical phenomena. It has applications in fundamental tests of quantum mechanics and photonic quantum information processing. The review covers theoretical and experimental progress in this area, including the creation of photonic entanglement, tests of quantum mechanics, quantum communication protocols, and quantum computation with linear optics. The review discusses the basics of interference and quantum entanglement, including classical and quantum interference, quantum entanglement, and interferometry with entangled two- and multi-photon states. It also covers photonic qubits and linear optics, experimental realizations of photonic entanglement, and the falsification of a realistic world view. The review discusses the applications of photonic entanglement in quantum communication, including quantum dense coding, quantum teleportation, entanglement swapping, and quantum memory and quantum repeaters. It also covers photonic quantum computing, including linear-optical two-qubit logic gates, cluster-state quantum computing, and few-photon quantum computing experiments. The review concludes with a discussion of the future of photonic quantum information processing, emphasizing the importance of further research in this area. The review is based on a comprehensive analysis of the literature on multi-photon interferometry and its applications in quantum mechanics and quantum information processing.