The publication in English of Karl R. Popper's The Logic of Scientific Discovery (London, Hutchinson, 1959) twenty-five years after the appearance of the German original, is an important event. At the time when many philosophers questioned the cognitive value of scientific hypotheses (and even of the most down-to-earth empirical generalizations), Popper has shown how to reconcile the free use of bold explanatory hypotheses with an insistence on empirical testing. Many of Popper's views have now been accepted even by most of his earlier opponents, which is no small tribute to a philosopher who had challenged an important current of opinion. But, though the basic aspects of Popper's theory have attracted considerable attention in recent years, some of his finer points have received insufficient discussion. It is hoped that the publication of this important book in English translation will now help to remedy this situation.