We summarize recent theoretical results dealing with the formation of massive stellar systems. The key premise is the high stellar density which leads to a very efficient star forming process in the so called positive feedback mode. Multiple implications arise from such models, e.g., much less mechanical energy exits the starburst process and much of the mass provided by massive stars is used to form more stars within the cluster's volume. Such a star forming mode needs to be considered in the formation of compact galaxies and stellar systems in galaxies (e.g., bulges and super star clusters); a star forming mode that leads to the rapid formation of massive and compact systems with multiple stellar generations with a high metallicity spread.