The masses of fermions and gauge bosons enter the Standard Model through the Higgs mechanism, which is satisfactory technically but is not understood physically. We do not know what nature really does to give mass to particles, nor what experimental clues will lead us to nature's solution. Understanding Higgs physics is necessary to complete the Standard Model, and to learn how to extend it and improve its foundations. This book is a collection of current work and thinking about these questions by active workers. Some chapters are speculations about what form the answers will take, and some update and extend previous books and reviews. Some chapters emphasize theoretical questions, some connections to other parts of physics, and some how we can get the data to teach us nature's solution.
