Metasurfaces can be opportunely and specifically designed to manipulate electromagnetic wavefronts for many applications. In recent years, a wide variety of metasurface-based optical devices such as planar lenses, beam deflectors, polarization converters, and so on have been designed and fabricated. Of particular interest are tunable metasurfaces, which allow the modulation of the optical response in real time, for instance, the variation in the focal length of a converging metalens. Response tunability can be achieved through external sources that modify the permittivity of the materials constituting the nanoatoms, the substrate, or both, as well as their geometrical arrangement. The modulation sources can be classified into electromagnetic fields, thermal sources, mechanical stressors, and electrical bias. These stimuli by modifying the relative permittivity tensor allow the tuning of the matamaterials optical properties. A big variety of tunable materials have been used in metasurfaces engineering, such as transparent conductive oxides, ferroelectrics, phase change materials, liquid crystals, and semiconductors. Metasurfaces tuning is very advantageous for applications spanning from basic to applied optics for communications, depth sensing, holographic displays, and biochemical sensors.