We review the main neutrino emission mechanisms in neutron star crusts andcores. Among them are the well-known reactions such as the electron-positronannihilation, plasmon decay, neutrino bremsstrahlung of electrons collidingwith atomic nuclei in the crust, as well as the Urca processes and neutrinobremsstrahlung in nucleon-nucleon collisions in the core. We emphasize recenttheoretical achievements, for instance, band structure effects in neutrinoemission due to scattering of electrons in Coulomb crystals of atomic nuclei.We consider the standard composition of matter (neutrons, protons, electrons,muons, hyperons) in the core, and also the case of exotic constituents such asthe pion or kaon condensates and quark matter. We discuss the reduction of theneutrino emissivities by nucleon superfluidity, as well as the specificneutrino emission produced by Cooper pairing of the superfluid particles. Wealso analyze the effects of strong magnetic fields on some reactions, such asthe direct Urca process and the neutrino synchrotron emission of electrons. Theresults are presented in the form convenient for practical use. We illustratethe effects of various neutrino reactions on the cooling of neutron stars. Inparticular, the neutrino emission in the crust is critical in setting theinitial thermal relaxation between the core and the crust. Finally, we discussthe prospects of exploring the properties of supernuclear matter by confrontingcooling simulations with observations of the thermal radiation from isolatedneutron stars.