This book and its prequel （Theories of Matter Space and Time： Classical theories） grew out of courses that we have both taught as part of the third and fourth year of the undergraduate degree program in Physics at Southampton University， UK. Our goal was to guide the full MPhys undergraduate cohort through some of the trickier areas of theoretical physics that we expect our undergraduates to master. In particular the aim is to move beyond the initial courses in classical mechanics， special relativity， electromagnetism and quantum theory to more sophisticated views of these subjects and their interdependence. Our approach is to keep the analysis as concise and physical as possible whilst revealing the key elegance in each subject we discuss.
　　In the first book we introduced several key areas of study. Firstly the principle of least action， an alternative treatment of Newtonian dynamics， that provides new
　　understanding of conservation laws. Secondly special relativity including four-vector notation. Thirdly we discussed the integral and differential forms of Maxwell's equations before massaging them to four-vector form so that the Lorentz boost properties of electric and magnetic fields were transparent.