A patent infringement suit is a federal action claiming that someone is practicing a patented invention without authority, and every such claim rises or falls on the patent's numbered claims and the all-elements rule. This guide explains what actually constitutes infringement and how claims and defenses operate in U.S. federal court, written for accused businesses, patent owners, and the counsel who advise them. It maps the types of infringement (direct under 35 U.S.C. 271(a); induced and contributory under 271(b)-(c); divided infringement after Akamai; and the special categories in 271(e), (f), and (g)), the decisive role of claim construction under Markman and Phillips, and literal infringement and the doctrine of equivalents under Graver Tank, Warner-Jenkinson, and Festo. It works through the full defense bench—non-infringement, invalidity under Sections 101, 102, 103, and 112, inequitable conduct, patent misuse, assignor estoppel, license and exhaustion, and the narrow personal defenses—and the remedies, from a Georgia-Pacific royalty and Panduit lost profits to enhanced damages under Halo, fees under Octane, and injunctions under eBay. A running hypothetical—a doorbell-camera startup that receives a cease-and-desist letter from a security-industry incumbent—keeps the doctrine concrete, and the guide closes with an FAQ, an authorities table, and practical takeaways.