This volume presents for the first time complete proofs of the fundamental arithmetic duality theorems that have come to play an increasingly important role in number theory and arithmetic geometry. Chapter 1 is devoted to an exposition of these theorems in the Galois cohomology of number fields announced by Tate in 1962 and describes later work in the same area. The discussion assumes only a knowledge of basic Galois cohomology and class field theory. Chapter 2 focuses on the work of Artin and Verdier who re-interpreted and developed Tate's ideas in the framework of etale cohomology; some of the more recent developments in this area are also covered. Finally, in Chapter 3, which contains a number of new results, it is shown how flat cohomology is needed in order to prove and to apply duality theorems in the case of groups which have torsion of order divisible by one of the residue characteristics.