Distributional semantics is the study of how distributional information can be used to model semantic facts. Its theoretical foundation has become known as the Distributional Hypothesis: Lexemes with similar linguistic contexts have similar meanings. This chapter presents the epistemological principles of distributional semantics. First, we explore the historical roots of the Distributional Hypothesis, tracing them in several different theoretical traditions, including European structuralism, American distributionalism, the later philosophy of Ludwig Wittgenstein, corpus linguistics, and behaviorist and cognitive psychology. Then, we discuss the place of distributional semantics in theoretical and computational linguistics.