During his long career, Rudolf Carnap held various different views about the concept of truth and its philosophical significance. As a good logical empiricist, he insisted on the distinction between logical and factual statements, and employed his technical powers to give rigorous characterizations of the notions of logical, analytic, and factual truth. The development of Carnap's views reflected his ability to quickly absorb new influences and his broad interests ranging from logic to epistemology, philosophy of language, and philosophy of science (Sections 1 and 2). Carnap was the co-founder of logical semantics with Alfred Tarski (Section 3), and therefore it is especially interesting to see how Carnap's work was related to Tarski's early definition of truth and to the later Tarskian model theory (Section 4). In Section 5, we discuss some difficulties in Carnap's liberally empiricist treatment of scientific theories. The final section 6 makes some remarks on Carnap's contemporary relevance.