This study looks at the basic principles of optical parametric processes and recent results on the rapidly developing optical parametric device technology. The theoretical basis of stimulated and spontaneous optical parametric processes and detailed design considerations of optical parametric oscillators and amplifiers are discussed, followed by a review of the materials properties of the most important nonlinear optical crystals for such applications. It concludes with a review of the recent developments on practical low-repetition rate nanosecond optical parametric oscillators and broadly tunable high-repetition rate continuous-pulse-train femtosecond optical parametric oscillations from the uv to the mid ir.