We introduce sub-picosecond transient four-wave-mixing techniques as a novel tool to optically study resonant tunneling in semiconductor heterostructures. The resonant carrier transfer in an asymmetric double quantum well structure is traced by monitoring the decay of the diffracted signal as a function of the applied electric field. The decay of the diffracted signal is much faster when the alignment of the electronic levels leads to resonant tunneling and scattering. © 1990.