In the catalyst layers, all of the various phases of a fuel cell exist. Thus, there is membrane, gas, liquid, and solid (mainly carbon). In addition, there are electrocatalysts where the charge-transfer reactions take place. An accurate model of a catalyst layer requires that all of these phases and their various interactions be modeled from a physical perspective. In this transaction, the macroscopic modeling approaches towards a catalyst layer will be discussed. Specifically, the modeling and multi-scaling of catalyst-layer phenomena will be introduced including accounting of Pt-oxide formation and non-unity reaction orders in a simultaneous reaction/diffusion framework. Of particular interest is the modeling and accounting of recently observed unexpected mass-transport limitations at very low Pt loadings.