Core-collapse supernovae (SNe) are the final act in the evolution of stars more massive than about 8–9 solar masses. Determining the progenitors of these explosive events and how massive stars are linked to the different SN types are topics of major significance for several fields of astrophysics. Recent progress in observational techniques now allow for rapid-response spectroscopic observations of SNe within a day of detection Gal-Yam et al. (2014). This allows the study of early phases when the SN shock front has not yet reached spatial scales of 1014 cm. Depending on the progenitor's wind density and SN shock front velocity, these early-time SN observations may probe epochs early enough that the dense parts of the progenitor wind and circumstellar medium (CSM) have not yet been overrun by the SN shock front.