Neutrino-neutrino scatterings create entanglements between them which may affect their flavor evolution. Although insignificant in terrestrial settings, this phenomenon may be consequential in some astrophysical environments where neutrinos transport significant amount of energy and lepton number including core collapse supernova and neutron star mergers. The problem is equivalent to that of a many-body system away from equilibrium and presents significant challenges. In this talk, I will summarize some approaches to calculating the neutrino flavor evolution, such as the path integral approach, and the Bogoliubov-Born–Green–Kirkwood–Yvon hierarchy approach.