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This paper reviews the theory and potential applications of magnetically variable delay lines and oscillators which employ a magnetostrictive film on a piezoelectric surface-acoustic-wave (SAW) substrate. Cases analyzed in detail indicate that the delay change arises mainly from a rotation of the magnetic moment from the films' easy axes toward the applied field direction; thus the interaction is essentially nondispersive. Use of amorphous metallic-glass overlays is particularly attractive because their high magnetostriction and low magnetic anisotropy makes a relatively large delay variation possible with a small change in bias field. Since the SAW velocity can be changed only by % or less with present film technology, applications are restricted to those where only a small frequency or delay adjustment is required. Two prototype examples are considered in detail--a variable delay line for steering an adaptive array antenna and a tunable resonator oscillator capable of tracking high speed Doppler targets. In both of these examples, the magnetic film/SAW substrate geometry is seen to be an attractive alternative to competitive approaches.