We will discuss the Makeenko--Migdal equation (MM equation) which relates variations of a "Wilson loop functional" (relative to the Euclidean Yang--Mills measure) in the neighborhood of a simple crossing to the associated Wilson loops on either side of the crossing. We will begin by introducing the 2d -- Yang-Mills measure and explaining the necessary background in order to understand the theorem. The goal is to describe the original heuristic argument of Makeenko and Migdal and then explain how these arguments can be made rigorous using stochastic calculus.