BRIAN R. MARTIN and GRAHAM SHAW have researched and taught for many years in the Physics and Astronomy departments at University College London and the University of Manchester, respectively. Prior to that they have held positions at various institutes, including Brookhaven National Laboratory and Niels Bohr Institute (Martin), Columbia University and Rutherford Laboratory (Shaw). They have previously collaborated on other successful textbooks, including Particle Physics (4th edn. 2017) and Mathematics for Physicists (2015), both published by Wiley. Brian has also published books on statistics and a Beginner's Guide to Particle Physics, while Graham is the co-author with Franz Mandl of the well-known postgraduate text, Quantum Field Theory (2nd. edn. 2010), also published by Wiley.