This text treats what is conventionally called electromagnetism or Maxwell's theory within the context of Gauge theory or Yang-Mills theory. A major theme of the book is that fields are not stand-alone entities but are defined by their boundary conditions. It has practical relevance to efficient antenna design, understanding forces and stresses in high energy pulses, ring laser gyros, high-speed computer logic elements, efficient transfer of power, parametric conversion and many other devices and systems. Conventional electromagnetism is shown to be an underdeveloped, rather than a completely developed, field of endeavour, with major challenges in development still to be met.