We present theoretical and experimental results for spin-dependent tunneling in ferromagnetic junctions. Microscopic calculation shows that direct tunneling in iron group systems leads to about a 30% change in resistance, close to observed values. Tunneling via polarized surface states may lead to enhanced tunnel magnetoresistance (TMR). Our previous prediction of a detrimental role of defects in the barrier is confirmed experimentally in the present work. The degradation of magnetoresistance with bias has been studied and shows universal features, attributed to effects of tunneling assisted by magnons and phonons. Non-linear features at low biases are related to excitation of bulk modes by tunneling electrons. The model presented here is applied qualitatively to half-metallics with 100% spin polarization (e.g., CrO₂/TiO₂ and CrO₂/RuO₂), where one-magnon processes are suppressed and the resistance change can be a few 1000 percent. We discuss a nature of colossal magnetoresistance as a consequence of a carrier density collapse during ferromagnetic transition in doped charge-transfer insulators La_1-xD_xMnO₃ (D = Ca, Sr, Ba) driven by exchange interaction of polaronic carriers with localized spins.

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