This volume is an English translation of "Cohomologie Galoisienne". The original edition （Springer LN5， 1964） was based on the notes， written with the help of Michel Raynaud， of a course I gave at the College de France in 1962-1963. In the present edition there are numerous additions and one suppression: Verdier's text on the duality of profinite groups. The most important addition is the photographic reproduction of R. Steinberg's "Regular elements of semisimple algebraic groups"， Publ. Math. I.H.E.S.， 1965. I am very grateful to him， and to I.H.E.S.， for having authorized this reproduction.