Abstract: Forum selection is hardly new, but courts still disagree on the basics. What do these agreements really do, and how should they be invoked? This Article suggests a few answers. First, forum selection is a form of procedural waiver. A permissive agreement waives the parties' objections to litigating in the chosen court. A mandatory one waives their rights to litigate somewhere else. Whether each agreement succeeds in waiving what it purports to waive is a question of procedure, not just contract law. So its validity rests on the procedural law of the forum -- including, in a federal forum, federal law. Second, forum selection can be raised as a defense. When a plaintiff files in the wrong court, a mandatory agreement gives that court a reason to deny recovery. Whatever other remedies are also available, such as venue transfer or forum non conveniens, the agreement can be invoked as an affirmative defense -- whether in the answer, on summary judgment, or (under the right circumstances) in a motion to dismiss. To some, these procedures may seem unwieldy; to others, unduly harsh. Perhaps we should handle forum selection in some other way. If so, we should amend our statutes or our Federal Rules. Until we do, though, we should use the rules we have -- under which forum selection is a type of waiver, and a defense.