Neutrinos are the second most ubiquitous Standard Model particles in the universe. On the other hand, they are also the ones least likely to interact. Connecting these two points suggests that when a neutrino is detected, it can divulge unique pieces of information about its source. Among the known neutrino sources, core-collapse supernovae in the universe are the most abundant for MeV-energies. On average, a single collapse happens every second in the observable universe and produces 10⁵⁸ neutrinos. The flux of neutrinos reaching the Earth from all the core-collapse supernovae in the universe is known as diffuse supernova neutrino background. In this Chapter, the basic prediction for the diffuse supernova neutrino background is presented. This includes a discussion of an average neutrino signal from a core-collapse supernova, variability of that signal due to the remnant formed in the process, and uncertainties connected to the other astrophysical parameters determining the diffuse flux, such as cosmological supernova rate. In addition, the current experimental limits and detection perspectives of diffuse supernova neutrino background are reported.