The classical circle method of Hardy and Littlewood is one of the most effective methods of additive number theory. Two examples are its success with Waring's problem and Goldbach's conjecture. In this book, Wang offers instances of generalizations of important results on diophantine equations and inequalities over rational fields to algebraic number fields. The book also contains an account of Siegel's generalized circle method and its applications to Waring's problem and additive equations and an account of Schmidt's method on diophantine equations and inequalities in several variables in algebraic number fields.