This library entry answers trademark law's threshold question: what kinds of things can serve as a mark, and which of them the law will actually protect. It maps the full universe of registrable subject matter—words, names, logos, slogans, trade dress, color, sound, scent, motion, and the specialized worlds of service, collective, and certification marks—and anchors each to the one idea that unifies the field: a designation is a trademark only insofar as it identifies a source. It works through the Abercrombie distinctiveness spectrum that sorts marks from generic to fanciful, the genericide and Booking.com problems that turn on consumer perception, and the functionality bar that keeps useful features inside the patent system. It closes with the Section 2 statutory bars as reshaped by the Supreme Court's Tam, Brunetti, and Elster trilogy. Invented hypotheticals and plain-language explanations make the doctrine usable by lawyers, founders, examiners, and judges alike.