Resonant second-order nonlinear optical processes in Quantum Cascade (QC) Lasers were analyzed. It was demonstrate that intersubband (IS) optical transitions in stacked semiconductor quantum wells (QWs) can function as a simultaneous source of fundamental pump and nonlinearity generated light. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10-15 nW of second-harmonic signal.