Quantum mechanical treatment of parametric processes including spontaneous parametric down conversionSpontaneous parametric down conversion (SPDC) (SPDCSpontaneous parametric down conversion (SPDC)) and parametric amplifier are discussed in this chapter. Expressions for the output quantum state of the SPDCSpontaneous parametric down conversion (SPDC) process is derived, and it is shown that at degeneracy the output state is a squeezed stateSqueezed state. Also discussed are techniques to achieve spectral separability of the output photon pair. Phase-sensitive and phase-insensitive parametric amplifier is analyzed showing how phase-sensitive parametric amplifiersParametric amplifiers can provide us with noise-free amplification.