Quantum mechanics is well approximated by classical physics when the Planck constant is considered very small, i.e., at the semi-classical limit. Typically, one can study an observable associated with a particle, such as its momentum or its position, and show that its dynamics is given by a development in powers of the Planck constant whose zeroth order corresponds to classical dynamics. In this talk, I will present more precisely the concept of semi-classical limit, the standard mathematical results known for non-relativistic quantum mechanics, and my work that concerns the semi-classical limit in the context of relativistic quantum mechanics. Concretely, I will show how to adapt the modulated energy method developed on the Schrödinger equation to the Klein-Gordon equation and how we recover relativistic mechanics (instead of classical mechanics) at the semi-classical limit.