Optical metasurfaces are rapidly gaining traction as ultrathin multifunctional platforms for light management. Their capability to shape the wavefront of light makes them ideal for free-space optical analog computing. In this framework, the fast reconfigurability of metasurfaces constitutes an indisputable requirement to secure practical application. To date, electro-optical and all-optical modulation represent the foremost approaches for light manipulation through metasurfaces, thanks to the compatibility with CMOS technology and the modulation rates above the GHz. Yet, fast modulation often comes at the expense of optical signals modulation depths (i.e. efficiency). The last decades witnessed major efforts in the field of nanophotonics to attain efficient nonlinear optical effects in nanoscale volumes. The main challenge in this scenario resides in the perturbative character of the nonlinear interactions in nanoscale systems. A key strategy to circumvent this limitation is to boost light–matter interaction by leveraging optical resonances in engineered nanoantennas and metasurfaces [1]. While conversion efficiencies at the nanoscale will unlikely reach those of bulk nonlinear crystals, the nonlinear character of these signals grants higher sensitivity to environmental changes compared to linear signals. This enables strong signal modulation depths at the nanoscale [2], fueling the development of nonlinear metasurfaces for electro- and all-optical light manipulation (e.g. modulation, steering and amplification). In this seminar, I will show our latest approaches to light modulation and routing by means of optical metasurfaces based on highly nonlinear materials, namely lithium niobate (LiNbO3) and aluminum gallium arsenide (AlxGa1-xAs). First, I will present a LiNbO3 metasurface design, empowered by quasi bound states in the continuum (q-BIC) resonances, that allows realizing electro-optic modulation of optical signals at telecom wavelengths with efficiency exceeding 10% at rates above the GHz [3]. Noteworthy, this platform also achieves a record modulation of the second harmonic generated by a CW pump laser of more than one order of magnitude. Finally, by means of a periodic AlxGa1-xAs metasurfaces, I will demonstrate all-optical routing and polarization modulation of upconverted optical signals exploiting nonlinear interferometry [4,5]. These experimental realizations allow envisioning modulation rates approaching that of the light optical cycle and can be employed for the realization of enhanced sensing platforms.