This volume has grown out of lectures given by Professor Pfister over many years. The emphasis here is placed on results about quadratic forms that give rise to interconnections between number theory, algebra, algebraic geometry and topology. Topics discussed include Hilbert's 17th problem, the Tsen-Lang theory of quasi algebraically closed fields, the level of topological spaces and systems of quadratic forms over arbitrary fields. Whenever possible proofs are short and elegant, and the author's aim was to make this book as self-contained as possible. This is a gem of a book bringing together thirty years' worth of results that are certain to interest anyone whose research touches on quadratic forms.