Universite Pierre et Marie Curie, Ecole Normale Superieure, CNRS 1 article published in JoVE Engineering Quantum State Engineering of Light with Continuous-wave Optical Parametric Oscillators Olivier Morin1, Jianli Liu1, Kun Huang1,2, Felippe Barbosa3, Claude Fabre1, Julien Laurat1 1Laboratoire Kastler Brossel, Université Pierre et Marie Curie, Ecole Normale Supérieure, CNRS, 2State Key Laboratory of Precision Spectroscopy, East China Normal University, 3Instituto de Física, Universidade de São Paulo We describe the reliable generation of non-Gaussian states of traveling optical fields, including single-photon states and coherent state superpositions, using a conditional preparation method operated on the non-classical light emitted by optical parametric oscillators. Type-I and type-II phase-matched oscillators are considered and common procedures, such as the required frequency filtering or the high-efficiency quantum state characterization by homodyning, are detailed.