This graduate text introduces relativistic quantum theory, emphasizing important applications in condensed matter physics. Relativistic quantum theory is the unification of Einstein's theory of relativity and the quantum mechanics of Bohr, Schrödinger and Heisenberg. Beginning with basic theory, the book then describes essential topics. It includes many worked examples and exercises as well as an extensive reference list. This clear account of a crucial topic will be valuable to graduates and researchers working in condensed matter physics and quantum physics.