A description is given of the development of new geometries for resonant tunneling devices involving tunneling through coupled quantum well states in a variably spaced superlattice energy filter (VSSEF). Resonant tunneling through high energy states in AlAs/GaAs VSSEF devices and excited states in AlGaAs/GaAs superlattices are demonstrated and their application to higher power microwave sources and resonant tunneling transistor described. Potential applications of resonant tunneling structures to optical devices such as avalanche detectors and infrared emitters are discussed.