We will describe model spaces, first in the classical Riemannian setting, and then in the sub-Riemannian setting. The notions of Carnot groups and nilpotentization will be introduced as a first step in the classification process of sub-Riemannian model spaces. The results in step 2 are known, and we will outline the classification of step 3 and rank 3 sub-Riemannian model spaces. This case provides insight into the higher dimensional classification as well as allowing for methods not available for sub-Riemannian model spaces with different rank or step.