This book is devoted to the numerous phenomena arising from the interplay between electromagnetic resonances and nonlinear optical interactions. These resonances are associated with surface plasmas or guided waves, excited in nonlinear optical resonators such as prisms or grating couplers. Topics include rigorous theories of diffraction by gratings in nonlinear optics, presented in a form ready for numerical implementations; scattering the matrix description in nonlinear optics leading to the phenomological approach based on the use of poles and zeros and other behaviours.