High-cadence long-term monitoring of the transient sky suggests that most core-collapse supernovae are influenced at some level by interaction with circumstellar material (CSM). Although considered so far a feature in core-collapse supernovae, this observation has led theorists to revise the idealized view of massive star evolution and explosion, and consider additional time-dependent, dynamical, and multi-dimensional effects that remain today poorly understood.

In this talk, I will review the basic features of massive star evolution, the ultimate gravitational collapse of their degenerate core, their explosion as supernova, and the subsequent evolution of the ejecta and radiation properties (i.e., light curves and spectra). I will then describe the variety of signatures that are suggestive of ejecta interaction with CSM and how we can model the radiation and the dynamics of these events. I will consider various configurations for the CSM, including when it is dense but confined to the direct vicinity of the star, when it is dense but extended, when the CSM is more massive than the ejecta that strike it, and finally when the CSM is very distant and gives rise to delayed interaction when the ejecta have expanded for a year or more.