The roadmap for silicon device technology has been drawn, extending to the year 2010, and featuring a CMOS transistor with a gate length of 0.07 μm . Beyond this point, silicon heterojunctions could provide a means to further device scaling. Silicon heterojunctions could also bring new devices to the silicon substrate including light emitters and detectors, and resonant tunneling diodes (RTDs). Today SiGe/Si and SiGeC/Si heterojunctions are receiving the greatest attention, but heterojunctions now being developed to realize silicon RTDs are increasing the heterojunction options for silicon-based quantum-well and optical devices. Here we outline the fundamental device requirements for silicon optical and tunneling devices and describe progress on silicon heterojunction development towards demonstration of silicon-based RTDs. Materials now under study include, ZnS, crystalline oxides and nitrides; new processes could provide methods for forming crystalline materials over amorphous barriers.