Dependence is a common phenomenon wherever one looks: ecological systems, astronomy, human history, stock markets -but what is the logic of dependence? This book is the first to carry out a systematic logical study of this important concept, giving on the way a precise mathematical treatment of Hintikka's independence friendly logic. Dependence logic adds the concept of dependence to first-order logic. Here the syntax and semantics of dependence logic are studied, dependence logic is given an alternative game-theoretic semantics, and sharp results about its complexity are proven. This is a textbook suitable for a special course in logic in mathematics, philosophy, and computer science departments, and contains over 200 exercises, many of which have a full solution at the end of the book. It is also accessible to readers with a basic knowledge of logic, interested in new phenomena in logic.