This paper reviews the basic physics of those cosmic rays which can affect terrestrial electronics. Cosmic rays at sea level consist mostly of neutrons, protons, pions, muons, electrons, and photons. The particles which cause significant soft fails in electronics are those particles with the strong interaction: neutrons, protons, and pions. At sea level, about 95% of these particles are neutrons. The quantitative flux of neutrons can be estimated to within 3×, and the relative variation in neutron flux with latitude, altitude, diurnal time, earth's sidereal position, and solar cycle is known with even higher accuracy. The possibility of two particles of a cascade interacting with a single circuit to cause two simultaneous errors is discussed. The terrestrial flux of nucleons can be attenuated by shielding, making a significant reduction in the electronic system soft-error rate. Estimates of such attenuation are made.