A polaron is a quasiparticle formed when a conduction electron polarizes the surrounding ionic lattice in a polar semiconductor or ionic crystal. The electron is dressed by a cloud of virtual phonons, increasing its effective mass m* = m_e / (1 − α/6) in the Fröhlich model (weak coupling). The dimensionless coupling constant α determines the regime: α ≪ 1 (weak, perturbative), α ~ 1-6 (intermediate), α ≫ 6 (strong, self-trapped). In the large-polaron limit the cloud extends over many lattice constants; strong coupling gives a small polaron localized on a single site. The effective mass, mobility, and optical absorption all depend critically on α.