  The W51 cloud complex is one of the best laboratories in our Galaxy to study
high-mass star formation. At a distance of about 5 kpc, it is the closest
region containing a high-mass protocluster, and it has two. The cloud includes
a long infrared-dark cloud, is interacting with a supernova remnant, and
contains a variety of unique massive protostellar sources. This article is an
observational review of the region.
