  The measurement of the anisotropy in the arrival direction of cosmic rays is
complementary to the study of their energy spectrum and chemical composition to
understand their origin and propagation. It is also a tool to probe the
structure of the magnetic fields through which cosmic rays travel. As cosmic
rays are mostly charged nuclei, their trajectories are deflected by the action
of galactic magnetic field they propagate through before reaching the Earth
atmosphere, so that their detection carries directional information only up to
distances as large as their gyro-radius. If cosmic rays below
$10^{15}{\rm\,eV}$ are considered and the local galactic magnetic field
($\sim3{\rm\,\mu G}$) is accounted for, gyro-radii are so short that isotropy
is expected. At most, a weak di-polar distribution may exist, reflecting the
contribution of the closest CR sources. However, a number of experiments
observed an energy-dependent \emph{"large scale"} anisotropy in the sidereal
time frame with an amplitude of about 10$^{-4}$ - 10$^{-3}$, revealing the
existence of two distinct broad regions: an excess distributed around
40$^{\circ}$ to 90$^{\circ}$ in Right Ascension (commonly referred to as
"tail.in" excess) and a deficit (the "loss cone") around 150$^{\circ}$ to
240$^{\circ}$ in Right Ascension. In recent years the Milagro and ARGO-YBJ
collaborations reported the of a "medium" scale anisotropy inside the tail-in
region. The observation of such small features has been recently claimed by the
IceCube experiment also in the Southern hemisphere. So far, no theory of cosmic
rays in the Galaxy exists which is able to explain the origin of these
different anisotropies leaving the standard model of cosmic rays and that of
the galactic magnetic field unchanged at the same time.
