Massive stars can shed mass in the months--years preceding stellar death, polluting their environment with dense circumstellar material. When these stars undergo core-collapse, the ensuing supernova explosion shocks this circumstellar material, powering thermal and/or non-thermal radiation across the electromagnetic spectrum. Emission of this kind has been successful in explaining radio supernovae, and is gaining renewed interest with the discovery of “extreme” cosmic explosions such as AT2018cow. In this talk I will present recent work modeling the thermal and non-thermal emission from such circumstellar interaction, its relation to observed (and predicted) events, and discuss the landscape of shock-powered transients.