Young massive clusters of & 105.5Mo show the extreme mode of star formation. They are common in the Early Universe and in starburst galaxies today. They are tremendously bright and powerful and can dominate the luminosity and energetics of the host galaxy. These clusters are born embedded in the obscuring gas and dust of giant molecular clouds. Very quickly–usually in less than 5 Myr–the active young stars disperse the remnant gas so the cluster emerges and is visible. In some clusters the stellar winds stall inside the cluster and continue to create multiple generations of stars from enriched material. In others, the stellar winds combine to form cluster super-winds that clear away the gas, distribute enriched gas through the host galaxy, and drive galactic-scale fountains and winds. We review recent observations and models of young clusters: those that are embedded and still accreting gas, others that are still embedded but beginning to emerge, and some that are driving strong winds back into intergalactic space.