This document discusses heat conduction through composite walls made of layers of different materials. It shows that the resistances to heat transfer of each layer can be combined into a total resistance. Specifically, it: 1) Derives equations for the heat transfer through each individual layer using Fourier's Law. 2) Integrates these equations over the thickness of each layer. 3) Combines the equations and applies boundary conditions at the surfaces to arrive at an expression relating the overall heat transfer to individual layer resistances. 4) Presents the famous formula showing that the overall heat transfer coefficient is equal to the additivity of individual layer resistances.