Transitions between the subbands of quantum wells have extremely large oscillator strengths. Since second-order nonlinear optical coefficients are proportional to the square of oscillator strength, strong nonlinear effects are expected in such systems, provided inversion symmetry is broken. This paper reviews the main results (optical rectification, second harmonic generation) obtained in structures in which the symmetry breaking has been realized by growing AlGaAs multiquantum wells with asymmetrical Al gradients. We show that, in these structures, the dipole matrix elements have magnitudes comparable to the well thicknesses, i.e. in the few nanometers range instead of the few picometers usually found in molecules. These Compositionally Asymmetrical multiquantum wells may thus be viewed as giant "quasi-molecules" optimized for optical nonlinearities in the mid infared.