We study, both experimentally and theoretically, the second-order nonlinear response from resonant metasurfaces composed of metal−dielectric nanodisks. We demonstrate that by exciting the resonant optical modes of the composite nanoparticles we can achieve strong enhancement of the second-harmonic signal from the metasurface. By employing a multipole expansion method for the generated second-harmonic radiation, we show that the observed SHG enhancement is due to the magnetic dipolar and electric quadrupolar second-order nonlinear response of the metasurface.T he nonlinear optical properties of nanostructures are known to differ substantially from those of bulk media because they are affected by strong confinement and local resonances. 1−7 It is well established that the strong field enhancement through formation of "hot spots" can dramatically boost nonlinear effects in metallic nanoparticles. 8−11 Importantly, in the case of metamaterials, the nanopatterning leads not only to more efficient nonlinear interaction but also to completely new nonlinear regimes due to the magnetic optical response of the constituent "meta-atoms". While exciting applications of the linear magnetic response of metamaterials for both metallic 12−16 and dielectric 17−19 structures have been readily achieved at infrared and even optical frequencies, the magnetic nature of nonlinear optical phenomena in metamaterials is still largely unexplored.We focus our attention on the process of second-harmonic generation (SHG), which is an even-order nonlinear process that vanishes in centrosymmetric materials in the electric dipole approximation. 20 For small nanoparticles, efficient SHG may be observed due to several factors, including local field enhancement, deviation of the particle shapes from a symmetric one, and surface effects. 21 Importantly, the resonances in plasmonic and dielectric nanoparticles, combined with a strong enhancement of the optical near-field, as well as the effective overlap of the interacting optical modes, allow for multifold enhancement of SHG. 22−24 The enhancement of efficiency of second-harmonic generation within extremely small nanoscopic volumes is of paramount interest in surface science, 25,26 colloidal chemistry, 27 and catalytic chemistry. 28 Due to its nature, second-harmonic generation is extremely sensitive to surface adsorbents. As such, even a single-molecule layer adsorbed onto a surface can completely change the surface nonlinear susceptibility. This sensitivity to changes of the chemical environment has found many applications to the study of the symmetry properties of surfaces, the nature of adsorbates at surfaces or interfaces, or noninvasive probing of buried interfaces.It was recognized that the local field distribution depends drastically on the size, shape, and mutual orientation of the nanoparticles, thus providing a way for the design of artificial materials with required nonlinear optical properties. 29−31 Such nonlinearity engineering resembles the engineering of nonlinear materials by che...