Nonlinear Optics is one of the most active fields for fundamental and applied research in physics. The interest for nonlinear optical phenomena is becoming extremely strong, because of their versatile and innovative properties and technological applications. We have developed a new first-principles theory, based on the Time-Dependent Density Functional Theory approach, for the calculation of the exact macroscopic second-order susceptibility. There two main goals in our formalism: 1) the exact relations between microscopic and macroscopic formulation of the second-order response function 2) a rigorous and straightforward inclusion of the many-body effects. As a test case, in order to validate our theory, I present Second-Harmonic Generation spectra for the cubic semiconductor GaAs, because it has been largely studied from experimentalists and some theoretical works exist. With our formalism we are able to give a conclusive comparison with experimental data, showing for the first time, excellent agreement with experimental data.