This material will be published by Cambridge University Press as “Information Theory” by Yury Polyanskiy and Yihong Wu. This prepublication version is free to view and download for personal use only. Not for redistribution, resale or use in derivative works. Note that while this version has matching equation numbers, theorems etc, the printed version will have many typos corrected. We recommend using it for thorough reading, while the free version could be used for quick reference lookup. © Polyanskiy & Wu 2023