In this seminar, I will discuss several hadronic interactions by means of non-perturbative techniques based on unitarization of amplitudes obtained from Chiral Perturbation Theory. First, I will discuss the scalar sector, $J^{PC}=0^{++}$, for which one needs to consider different meson-meson amplitudes. These are obtained from the unitarization of chiral amplitudes, avoiding {\it ad-hoc} parameterizations. We include the relevant channels up to $\sqrt{s}=$ 2 GeV, so as to reproduce all the scalar resonances appearing up to that energy. Later, I will consider the scattering of scalar ($f_0,a_0$) and pseudoscalar ($\pi$, $K$, $\eta$, $\eta'$) mesons, in order to study the possible appearence of heavier pseudoscalar resonances. Given the dynamically generated nature of the light scalars, these amplitudes can also be obtained from Chiral Lagrangians. Finally, I will elaborate further on the issue of the nature of the $\sigma$ meson, by studying its quadratic scalar radius. Given the small value found for the latter, it can be argued that a four-quark picture for the latter is more appropiate than a molecular one.