For many contemporary photonic technologies, ranging from mobile cameras, to light detection and ranging (LiDAR) and mixed-reality displays, wavefront shaping elements (e.g. lenses and waveplates) that are compact and tunable have become increasingly important. Recently, optical metasurfaces composed of judiciously-engineered nanostructures have provided an ultrathin and lightweight platform for modulating electromagnetic fields. All-dielectric metasurfaces have enabled high-performance optically-thin waveplates, beam steerers, and lenses, by virtue of precisely engineered resonant modes and low nonradiative losses. However, most metasurfaces have fixed functionalities after fabrication, restricting their potential practical applications. In this thesis, I develop several reconfigurable all-dielectric metasurfaces that act as ultrathin tunable optical modulators. Each proposed metasurface consists of arrays of silicon or germanium nanobars, engineered to support resonances sensitive to temperature or electric fields. In the first part of this thesis, Chapter 2 presents a germanium metasurface that acts as a thermally-actuated polarization converter. Its successful implementation relies on an anisotropic metasurface design that facilitates the thermo-optic tuning of a sharply-resonant spectral mode. By manipulating the temperature-dependent phase retardance between the two principal linear polarization states, a wide range of output polarization states are generated and controlled. In the next application, Chapters 3 and 4, I describe how metasurfaces can be merged with a well-studied electro-optic technology – liquid crystals (LCs) – to create a new class of voltage-controlled varifocal metalenses. The design exploits the electro-optic properties of LCs to tailor the local phase response of individual nanostructures, resulting in real-time modulations to the metalens focal length. Finally, in Chapter 5 I introduce a new type of simplified multi-color flat lens that reuses a small number of nanostructure types regardless of lens diameter. The proposed architecture may find future use in minimizing the computation cost and fabrication complexity of tunable multicolor metalenses. In full, by detailing the numerical optimization and experimental demonstration of several active metasurface platforms and showcasing their utility as tunable polarization converters and multifunctional focusing elements, this thesis opens new avenues for achieving ultracompact dynamic optical modulators for use in free space and integrated photonics.