  Modern photometric surveys of the sky suggest that many, perhaps most
supernovae (SNe) associated with the explosion of massive stars are influenced
at an appreciable level by their interaction with circumstellar material (CSM).
The photometric and spectroscopic diversity of these transients point to a wide
range of CSM properties in terms of mass, extent, composition, and location
relative to the exploding star, suggesting progenitors that cover from standard
to the most extreme mass loss rates. Surveys at high-cadence catch massive
stars at shock breakout and inform us on the immediate mass loss history before
core collapse. In contrast, long-term monitoring of these SNe cover the
transition to the birth of a SN remnant and document the progenitor mass loss
that took place centuries to millennia before explosion. Interacting SNe are
therefore not just extraordinary astrophysical laboratories to study
radiation-dominated shocks and probe the distant Universe, they also open the
path to novel and fundamental studies on stellar evolution, stellar stability,
or mass loss in single and binary massive stars.
