In the late 20th century, cosmology became a precision science. Now, at the beginning of the next century, the parameters describing how our universe evolved from the Big Bang are generally known to a few percent. One key parameter is the total mass density of the universe. Normal matter constitutes only a small fraction of the total mass density. Observations suggest this additional mass, the dark matter, is cold (that is, moving nonrelativistically in the early universe) and interacts feebly if at all with normal matter and radiation. There's no known such elementary particle, so the strong presumption is the dark matter consists of particle relics of a new kind left over from the Big Bang. One of the most important questions in science is the nature of this dark matter. One attractive particle dark-matter candidate is the axion. The axion is a hypothetical elementary particle arising in a simple and elegant extension to the standard model of particle physics that nulls otherwise observable CP-violating effects (where CP is the product of charge reversal C and parity inversion P) in quantum chromo dynamics (QCD). A light axion of mass 10 −(6-3) eV (the invisible axion) would couple extraordinarily weakly to normal matter and radiation and would therefore be extremely difficult to detect in the laboratory. However, such an axion is a compelling dark-matter candidate and is therefore a target of a number of searches. Compared with other particle dark-matter candidates, the plausible range of axion dark-matter couplings and masses is narrowly constrained. This focused search range allows for definitive searches, where a nonobservation would seriously impugn the dark-matter QCD-axion hypothesis. Axion searches use a wide range of technologies, and the experiment sensitivities are now reaching likely dark-matter axion couplings and masses. This article is a selective overview of the current generation of sensitive axion searches. Not all techniques and experiments are discussed, but I hope to give a sense of the current experimental landscape of the search for dark-matter axions.What Makes Up the Dark Matter? One of the great discoveries of the last century is that the vast majority of all matter in the universe is made of something other than dust, planets, gas, etc., which is the ordinary matter we see around us. From observations, we now have an accurate accounting of the dark-matter fraction of the universe at around one-quarter of its total energy density, with almost all of the remaining mass and energy density of the universe being the mysterious "dark energy." From Milky Way galactic observations, our nearby dark-matter density is around one-half of proton mass per cubic centimeter.Observational constraints imposed by structure formation in the Universe plus the remnant light-isotope abundance from primordial nucleosynthesis suggest the dark matter is a new exotic particle relic left over from the Big Bang. Interestingly, light axions, with masses much less than that of the electron, have dark matter-like p...