This course provides a review of optical parametric oscillators (OPOs), from basic operation principles to advanced architectures. The course will begin with a description of the fundamental concepts in nonlinear optics and frequency conversion, followed by a discussion of OPO devices and an overview of the current status of OPO technology and their applications. The discussion will mainly cover OPO systems operating in continuouswave (CW), picosecond and ultrafast femtosecond timescales. Specifically, the students gain knowledge of the basic principles of parametric generation and amplification; OPO design issues, nonlinear material and pump laser selection criteria, birefringent and quasiphasematching, OPO resonance configurations, OPO operating regimes; CW OPOs, including highpower singlyresonant oscillators, threshold conditions and tuning behavior, frequency control, CW OPOs for the visible, fiberpumped CW OPOs; picosecond OPOs, including highrepetitionrate cw and pulsed synchronouslypumped OPOs, fiberlaserpumped OPOs, quasiphasematched devices; femtosecond OPOs, including Ti:sapphirepumped OPOs, spectral and temporal control, femtosecond OPOs for the visible, UV and infrared; commercial development of OPO technology; and novel applications of OPOs in science and technology.