  Observations of molecular clouds in the gamma ray domain provide us with a
tool to study the distribution of cosmic rays in the Galaxy. This is because
cosmic rays can penetrate molecular clouds, undergo hadronic interactions in
the dense gas, and produce neutral pions that in turn decay into gamma rays.
The detection of this radiation allows us to estimate the spectrum and
intensity of cosmic rays at the cloud's position. Remarkably, this fact can be
used to constrain the cosmic ray diffusion coefficient at specific locations in
the Galaxy.
