The phase-field method to modeling microstructure evolution in materials is introduced in this chapter. It can be employed to model, understand, and predict complex material behaviors at meso-scale (on the order of nano- to micro-meter size), and it covers application-relevant time scales (on the order of seconds to years). Coarse-grained from the underlying atomic-level physics, it is an established approach to fill the length-scale gap between atomic/electronic scale calculations and the macroscopic continuum method, ignoring the underlying material’s inhomogeneity. Employing energy functionals and nonlinear partial differential equations, the general formulation of phase-field models is outlined. Several applications of the phase-field model are presented.