We illustrate the general scheme of the Sum Rule (SR) method using 2D Quantum Harmonic Oscillator (2DQHO) as a toy model. We introduce correlator, related to Green function of 2DQHO, and describe the property of Asymptotic Freedom for 2DQHO. We explain how the duality conception allows one to describe excited states. Finally we present numerical results and extract some lessons to learn from our exposition. Then we switch to the QCD and show that QCD SRs supply us the method to study hadrons in non-perturbative QCD.

PACS numbers: 11.55.Hx, 12.38.Lg