The contribution of neutrino-nucleon interactions to the neutrino opacity of matter is studied, special attention being paid to possible astrophysical applications such as supernova explosions. The results of recent accelerator experiments with high-energy neutrinos are used to show that nonresonant neutrino-nucleon scattering does not make a significant contribution to the neutrino opacity for astrophysically important conditions. The results of deep-mine cosmic-ray studies are then used to show that, (a) there are no resonances in the ν_μ-nucleon and ν _μ-nucleon systems with masses less than 60 BeV (laboratory neutrino energies <2×10^(+3) BeV), and (b) there are no resonances in the ν_(e)-neucleon and ν_(e)-nucleon systems with masses less than 7 BeV (laboratory neutrino energies <30 BeV). Neutrino absorption by bound nucleons is also discussed and a sum rule is proved for neutrino capture that is sufficiently accurate for most astrophysical applications. The effect of the exclusion principle on the capture cross sections is described and some applications to specific nuclei are presented. The accelerator experiments with high-energy neutrinos are then used to show that neutrino radioactivity, i.e., nuclear de-excitation by emission of a neutrino-antineutrino pair, is a substantially less important mechanism for stellar energy loss than was suggested by some previous estimates.