Potential theory for the constant coefficient PDE such as the Laplace equation is well-known and can be easily found in most standard PDE textbooks. However, potential theory for the variable coefficient PDE is much less well-known and often ignored. In this talk, I will derive some basic facts of the potential theory, including the continuity/discontinuity and spectrum properties of the single and double layer potentials. I will also discuss application of the potential theory for variable coefficient PDEs.