This paper surveys recent remarkable progress in the study of potential theory for symmetric stable processes. It also contains new results on the two-sided estimates for Green functions, Poisson kernels and Martin kernels of discontinuous symmetric α-stable process in boundedC1,1 open sets. The new results give explicit information on how the comparing constants depend on parameter α and consequently recover the Green function and Poisson kernel estimates for Brownian motion by passing α ↑ 2. In addition to these new estimates, this paper surveys recent progress in the study of notions of harmonicity, integral representation of harmonic functions, boundary Harnack inequality, conditional gauge and intrinsic ultracontractivity for symmetric stable processes. Here is a table of contents. 1. Introduction 2. Green function and Poisson kernel estimates 2.1. Estimates on balls 2.2. Estimates on boundedC1,1 domains 2.3. Estimates on boundedC1,1 open sets 3. Harmonic functions and integral representation 3.1. Two notions of harmonicity 3.2. Martin kernel and Martin boundary 3.3. Integral representation and uniqueness 3.4. Boundary Harnack principle 3.5. Conditional process and its limiting behavior 4. Conditional gauge and intrinsic ultracontractivity