The aim of this book is to provide a pedagodical yet comprehensive review of various aspects of four-dimensional Chern-Simons theory, a quantum field theory that has emerged as an organizing principle for various integrable systems, such as integrable spin chains, integrable lattice models, and two-dimensional integrable quantum field theories. Among topics covered are the derivation of the Yang-Baxter equation and its solutions, the Yangian algebra, realizations of two-dimensional integrable field theories and their Lax operators, string-theoretic embeddings of four-dimensional Chern-Simons theory, holographic duals of the theory, as well as dualities of integrable quantum field theories.