This guide covers the law governing online intermediaries and the content they host, organized around the two statutory regimes that do most of the work. Part one covers Section 230, its two operative provisions, and its four exclusions. Part two covers where the immunity ends, including the material contribution doctrine and the platform's own conduct. Part three covers the copyright safe harbor, its conditions, and the notice and counter-notice procedure. Part four covers content removal by category, since the available remedy depends entirely on what kind of content it is. Part five covers account termination, terms of service, and the contract claims that survive Section 230. Part six covers state action, official accounts, and government pressure on platforms. Part seven covers unmasking anonymous speakers and the procedural requirements. Part eight covers operator obligations, and part nine covers the unsettled questions and practice checklists for users, targets, and operators.