This monograph is on interpolation and definability, a notion central in pure logic and with significant meaning and applicability in all areas where logic is applied, especially computer science, artificial intelligence, logic programming, philosophy of science and natural language. Suitable for researchers and graduate students in mathematics, computer science and philosophy, this is the latest in the prestigious world-renowned Oxford Logic Guides, which contains Michael Dummet's Elements of Intuitionism (Second Edition), J.M. Dunn and G. Hardegree's Algebraic Methods in Philosophical Logic, H. Rott's Change Choice and Inference: A Study of Belief Revision and Nonmonotonic Reasoning, P.T. Johnstone's Sketches of an Elephant: A Topos Theory Compendium: Volumes 1 and 2, and David J. Pym and Eike Ritter's Reductive Logic and Proof Search: Proof, Theory, Semantics and Control.