  We present the results of an extensive observational study of the active
star-forming complex W51 that was observed in the J=2-1 transition of the 12CO
and 13CO molecules over a 1.25 deg x 1.00 deg region with the University of
Arizona Heinrich Hertz Submillimeter Telescope. We use a statistical
equilibrium code to estimate physical properties of the molecular gas. We
compare the molecular cloud morphology with the distribution of infrared (IR)
and radio continuum sources, and find associations between molecular clouds and
young stellar objects (YSOs) listed in Spitzer IR catalogs. The ratios of CO
lines associated with HII regions are different from the ratios outside the
active star-forming regions. We present evidence of star formation triggered by
the expansion of the HII regions and by cloud-cloud collisions. We estimate
that about 1% of the cloud mass is currently in YSOs.
