It has been long realized in theoretical studies that the first detectable electromagnetic signals of core-collapse supernovae (SNe) correspond to a transient phenomenon called shock breakout. Except for the initial neutrino burst, the explosion can hardly be perceived by an outside observer as long as the outward-propagating explosion shock is still trapped inside the progenitor star. However, it is predicted that when the optical depth between the front of the radiation-dominated shock and progenitor surface drops below that of the front region itself, which is roughly 10-20 as the shock reaches close to the surface, the high-temperature radiation first from a “hot tongue’’ and subsequently from the post-shock region begins to escape, largely in the form of UV and soft X-ray photons, and hence the star will be suddenly brightened by several orders. The duration of a shock breakout must be very short, determined by both the diffusion timescale and an observer time-lagging effect, and depending on the progenitor radius it can be about 1000s for a typical extended red supergiant, about one order shorter for a compact blue supergiant, and only about 10s for a Wolf-Rayet star.

Shock breakouts can be used to make constraints on important physical properties of core-collapse SNe that are otherwise difficult to obtain. For example, the duration, radiation temperature, and flux of a shock breakout all strongly depend on the progenitor radius, the information of which and other progenitor characteristics tends to quickly lose during the explosion following it. The radius is directly related to the progenitor type,making connections between the late-stage evolution of massive stars and SN mechanisms. The spectral and light curve data of a shock breakout can also reveal the outmost atmosphere structure of the progenitor as well as its proximate circumstellar environment and help understand the mass-loss behavior, one main uncertainty in late-stage evolution theories of massive stars. Remarkable explosion asymmetry that are widely supposed to be a necessity to achieve successive SN explosions in numerical simulations will also leave its imprints on the behaviors of shock breakout. In addition, any detection of very high-energy gamma-rays or high-energy neutrinos that have been suggested to accompany shock breakouts will also help constrain properties of the progenitor and its circumstellar environment.