Polarons are quasiparticles formed when electrons interact with the lattice vibrations in a polarizable crystal. They are essential to describe semiconductor transport properties and also arise in other systems, for instance, in transition metal oxides, colossal magnetoresistance, organic polymers, DNA transport. Moreover, the coupling between polarons has been one of the hallmarks of high-temperature superconductivity. However, a full microscopic description of these quasiparticles in real materials is challenging because real materials are imperfect and hard to model.