Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30&percent; change in resistance, which is close but lower than experimentally observed values. The larger observed values of the TMR might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half metallics with 100&percent; spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000&percent;. Examples of half-metallic systems areCrO2/TiO2andCrO2/RuO2.©1998 American Institute of Physics.