The axion is a hypothetical elementary particle whose existence would explain the baffling absence of CP violation in strong interactions. Its properties in addition make it a good dark matter candidate. Even though dark-matter axions would account for the overwhelming majority of mass in the universe, they are extraordinarily difficult to detect. Undaunted, groups around the world deploy exquisitely sensitive axion detectors. Each of these detectors is an elegant instrument and beyond state-of-the-art in novel ways. This colloquium will review the role of the axion in particle and astrophysics and will explore the various methods employed in the search.