 Rudolf Carnap (1891–1970) was one of the best-known philosophers of the twentieth century. Notorious as one of the founders, and perhaps the leading philosophical representative, of the movement known as logical positivism or logical empiricism, he was one of the originators of the new field of philosophy of science and later a leading contributor to semantics and inductive logic. Though his views underwent significant changes at various points, he continued to reaffirm the basic tenets of logical empiricism, and is still identified with it. His influence declined, therefore, when logical empiricism lost its dominance in the 1950s and 60s, even though many of the efforts of the next philosophical generation (such as Quine’s) may be understood as responses to Carnap. Beginning in the 1980s, a reassessment set in that has resulted in a much more nuanced and complex picture of his philosophy and its development. The literature on him is now enormous and still growing rapidly, and his ideas are presently enjoying a major revival in various areas of philosophy.