  The gas surrounding galaxies outside their disks or interstellar medium and
inside their virial radii is known as the circumgalactic medium (CGM). In
recent years this component of galaxies has assumed an important role in our
understanding of galaxy evolution owing to rapid advances in observational
access to this diffuse, nearly invisible material. Observations and simulations
of this component of galaxies suggest that it is a multiphase medium
characterized by rich dynamics and complex ionization states. The CGM is a
source for a galaxy's star-forming fuel, the venue for galactic feedback and
recycling, and perhaps the key regulator of the galactic gas supply. We review
our evolving knowledge of the CGM with emphasis on its mass, dynamical state,
and coevolution with galaxies. Observations from all redshifts and from across
the electromagnetic spectrum indicate that CGM gas has a key role in galaxy
evolution. We summarize the state of this field and pose unanswered questions
for future research.
