  The interplay between the formation of stars, stellar feedback and cloud
properties strongly influences the star formation history of giant molecular
clouds. The formation of massive stars leads to a variety of stellar clusters,
ranging from low stellar density OB associations to dense, gravitationally
bound starburst clusters. We aimed at identifying the massive stellar content
and reconstructing the star formation history of the W51 giant molecular cloud.
We performed near-infrared imaging and K-band spectroscopy of the massive stars
in W51. We analyzed the stellar populations using colour-magnitude and
colour-colour diagrams and compared the properties of the spectroscopically
identified stars with stellar evolution models. We derived the ages of the
different sub-clusters in W51 and, based on our spectroscopy derived an age for
W51 of 3 Myrs or less. The age of the P Cygni star LS1 and the presence of two
still forming proto-clusters suggests that the star formation history of W51 is
more complex than a single burst. We did not find evidence for triggered star
formation and we concluded that the star formation in W51 is multi seeded. We
finally concluded that W51 is a OB association where different sub-clusters
form over a time span of at least 3-5 Myrs.
