  Direct detection experiments, which are designed to detect the scattering of
dark matter off nuclei in detectors, are a critical component in the search for
the Universe's missing matter. The count rate in these experiments should
experience an annual modulation due to the relative motion of the Earth around
the Sun. This modulation, not present for most known background sources, is
critical for solidifying the origin of a potential signal as dark matter. In
this article, we review the physics of annual modulation, discussing the
practical formulae needed to interpret a modulating signal. We focus on how the
modulation spectrum changes depending on the particle and astrophysics models
for the dark matter. For standard assumptions, the count rate has a cosine
dependence with time, with a maximum in June and a minimum in December.
Well-motivated generalizations of these models, however, can affect both the
phase and amplitude of the modulation. We show how a measurement of an annually
modulating signal could teach us about the presence of substructure in the
Galactic halo or about the interactions between dark and baryonic matter.
Although primarily a theoretical review, we briefly discuss the current
experimental situation for annual modulation and future experimental
directions.
