During thee last decade millimeter vvave observations of ihe CO molecule have proved to be one of the most important probes
of the large scale structure and kinematics of the galactic disk. CO surveys show a strong concentralion of molecular gas in
the galactic center region and at a distance of 0.4 - 0.8 Ro often recorded to as the ‘galactic ring’. The shape of the CO radial
distribution is similar to nearly all other tracers of population I material, but quite different from the flat distribution of HI.
Recent high-resolution surveys allow an unprecedented view of individual molecular clouds throughout the disk on all size scales
between a few parsecs and many kiloparsecs. Massive, giant molecular clouds (CMC) are the dominant reservoir of interstellar
matter in the inner galaxy, and are the main active star forming component of the interstellar medium. This article reviews the
theoretical and empirical basis for using the 2.6 mm CO emission line as a tracer of H2 and summarizes the galactic distribution
and properties of the molecular clouds.