  Stars form within molecular clouds but our understanding of this fundamental
process remains hampered by the complexity of the physics that drives their
evolution. We review our observational and theoretical knowledge of molecular
clouds trying to confront the two approaches wherever possible. After a broad
presentation of the cold interstellar medium and molecular clouds, we emphasize
the dynamical processes with special focus to turbulence and its impact on
cloud evolution. We then review our knowledge of the velocity, density and
magnetic fields. We end by openings towards new chemistry models and the links
between molecular cloud structure and star--formation rates.
