  Almost 30 years have passed since the successful detection of supernova
neutrinos from SN 1987A. In the last decades, remarkable progress has been made
in neutrino detection technique, through which it may be possible to detect
neutrinos from a new source, pre-supernova (pre-SN) neutrinos. They are emitted
from a massive star prior to core bounce. Because neutrinos escape from the
core freely, they carry information about the stellar physics directly. Pre-SN
neutrinos may play an important role in verifying our understanding of stellar
evolution for massive stars. Observations of pre-SN neutrinos, moreover, may
serve as an alarm regarding a supernova explosion a few days in advance if the
progenitor is located in our vicinity, enabling us to observe the next galactic
supernova. In this review, we summarize the current status of pre-SN neutrino
studies from both of the theoretical and observational points of view.
