A survey of classical results on harmonic maps

Harmonic maps are nonlinear extensions of harmonic functions. They are critical points of natural energy functionals between Riemannian manifolds. Such type of problems appear in Physics, Geometry of Finance and the study of regularity and singularity uses methods from elliptic PDE, calculus of variations and geometric measure theory. In this paper, we present a general review of harmonic maps. It is a survey that aims to cover the main classical known results regarding the harmonic maps. We present results for the regularity, blow-ups and rectifiability for local minimizers, stationary harmonic maps and weakly harmonic maps.