A magnetic tunnel junction (MTJ), which consists of a thin insulating layer (a tunnel barrier) sandwiched between two ferromagnetic electrode layers, exhibits tunneling magnetoresistance (TMR) due to spin-dependent electron tunneling. Since the discovery of room-temperature (RT) TMR effect in 1995, MTJs with an amorphous aluminum oxide (Al–O) tunnel barrier have been studied extensively. The Al–O-based MTJs exhibit magnetoresistance (MR) ratios up to about 70% at RT and are currently used in the read heads of hard disk drives and magnetoresistive random access memory (MRAM). MTJs with MR ratios significantly higher than 70% at RT, however, are needed for next-generation spintronic devices. In 2001, first-principle theories predicted that the MR ratios of epitaxial Fe/MgO/Fe MTJs with a... crystalline MgO(001) barrier would be over 1000% because of the coherent tunneling of fully spin-polarized Δ1 electrons. In 2004, MR ratios of about 200% were obtained at RT in MTJs with a single-crystal MgO(001) barrier or a textured MgO(001) barrier. CoFeB/MgO/CoFeB MTJs for practical applications were also developed and found to have MR ratios up to 500% at RT. MgO-based MTJs are of great importance not only for device applications but also for clarifying the physics of spin-dependent tunneling.