  Low energy cosmic rays (up to the GeV energy domain) play a crucial role in
the physics and chemistry of the densest phase of the interstellar medium.
Unlike interstellar ionising radiation, they can penetrate large column
densities of gas, and reach molecular cloud cores. By maintaining there a small
but not negligible gas ionisation fraction, they dictate the coupling between
the plasma and the magnetic field, which in turn affects the dynamical
evolution of clouds and impacts on the process of star and planet formation.
The cosmic-ray ionisation of molecular hydrogen in interstellar clouds also
drives the rich interstellar chemistry revealed by observations of spectral
lines in a broad region of the electromagnetic spectrum, spanning from the
submillimetre to the visual band. Some recent developments in various branches
of astrophysics provide us with an unprecedented view on low energy cosmic
rays. Accurate measurements and constraints on the intensity of such particles
are now available both for the very local interstellar medium and for distant
interstellar clouds. The interpretation of these recent data is currently
debated, and the emerging picture calls for a reassessment of the scenario
invoked to describe the origin and/or the transport of low energy cosmic rays
in the Galaxy.
