Experimental data and theoretical explanation reported for spin polarized tunneling in ferromagnet/insulator/ferromagnet are reviewed. The magnetoresistance (MR) ratio due to the spin polarized tunneling is relatively smaller value than that of giant magnetoresistance in artificial superlattices. However, recent results reported by us are an order of magnitude larger than those reported in the past. We demonstrate that the magnetoresistance ratio is roughly proportional to the product of spin polarizations of both ferromagnets. The dependence of MR ratio, saturation resistance and conductance on temperature are also discussed.