Any student working with the celebrated Feynman Lectures will ?nd a chapter in it with the intriguing title Electromagnetic Mass [2, Chap. 28]. In a way, it looks rather out of date, and it would be easy to skate over it, or even just skip it. And yet all bound state particles we know of today have electromagnetic mass. It is just that we approach the question differently. Today we have multiplets of mesons or baryons, and we have colour symmetry, and broken ?avour symmetry, and we think about mass and energy through Hamiltonians. This book is an invitation to look at all these modern ideas