  The strong CP problem and its resolution through the existence of an axion
are briefly reviewed. The constraints on the axion from accelerator searches,
from the evolution of red giants and from supernova SN1987a combine to require
$m_a < 3 \cdot 10^{-3}$ eV, where $m_a$ is the axion mass. On the other hand,
the constraint that axions do not overclose the universe implies $m_a \gtwid
10^{-6}$ eV. If $m_a \sim 10^{-5}$ eV, axions contribute significantly to the
cosmological energy density in the form of cold dark matter. Dark matter axions
can be detected by resonant conversion to microwave photons in a cavity
permeated by a static magnetic field and tuned to the axion mass. Experiments
using this effect are described, as well as several other types of axion
searches.
