  How starburst clusters form out of molecular clouds is still an open
question. In this article, I highlight some of the key constraints in this
regard, that one can get from the dynamical evolutionary properties of dense
stellar systems. I particularly focus on secular expansion of massive star
clusters and hierarchical merging of sub-clusters, and discuss their
implications vis-a-vis the observed properties of young massive clusters. The
analysis suggests that residual gas expulsion is necessary for shaping these
clusters as we see them today, irrespective of their monolithic or hierarchical
mode of formation.
