Ferromagnetic-insulator-ferromagnetic tunneling has been measured in CoFe /Al2O3 /Co or NiFe junctions. At 295, 77, and 4.2 K the fractional change in junction resistance with magnetic field, ∆R/R, is 11.8%, 20%, and 24%, respectively. The value at 4.2 K is consistent with Julliere's model based on the spin polarization of the conduction electrons of the magnetic films. ∆R/R changes little with a small voltage bias, whereas it decreases significantly at higher bias \(>0.1 V\), in qualitative agreement with Slonczewski's model. These junctions have potential use as low-power field sensors and memory elements.