  The axion is a highly motivated elementary particle which could address two
fundamental questions in physics - the strong CP problem and the dark matter
mystery. Experimental searches for this hypothetical particle have started to
reach theoretically interesting sensitivity levels, particularly in the $\mu$eV
(GHz) region. They rely on large volume solenoid magnetic fields and microwave
resonators with signals read out by quantum noise limited amplifiers.
Concurrently, there have been intensive experimental efforts to widen the
search range by devising various techniques as well as to enhance sensitivities
by implementing advanced technologies. The developments and improvements in
these orthogonal approaches will enable us to explore most of the parameter
space of the axion and axion-like particles within the next couple of decades,
with the 1-25 GHz frequency range to be conquered well within the first decade.
We review the experimental aspects of axion physics and discuss the past,
present and future of the direct search programs.
