Massive clusters seem to be characterized by a burst of very rapid
star formation the final result of which is a rich population of stars
spanning the whole range of masses from the hydrogen burning limit to
the highest masses known.
They were probably the first clusters to form in the universe and the
preferred method of massive star formation even today. They may even
be the progenitors of the globular clusters. In a concerted effort to
better understand the origin and physical evolution of such clusters,
a large observing program was undertaken with the new panchromatic
camera on the recently refurbished HST. Massive clusters in a broad
range of sizes and distances were studied in some detail from local
ones in the MW to those in S0 galaxies in Virgo.  In this talk, I will
concentrate on the results of observations of the fully resolved
clusters in the local group like NGC 3603 and 30 Doradus.  The
observations allow for the first time to temporally resolve the
"starburst".
I will also discuss some of the implications of these findings on the
physics of star formation in such clusters and the significance of the
stellar IMF.