This chapter discusses resonant-tunneling (RT) double barrier (DB) structures important for their device potential or physical interest. New phenomena arising from charge accumulation in the quantum well as well as RT electron spectroscopy are treated. The chapter describes resonant tunneling bipolar transistors (RTBTs) with a double barrier in the base region. Design considerations for RTBTs with ballistic injection are also discussed and the observation of minority electron ballistic RT is presented. The chapter also describes RTBTs using thermionic injection and exhibiting high peak-to-valley ratio at room temperature in their transfer characteristics. It discusses two- and three-terminal RT devices with multiple peaks in the current-voltage characteristics. Some circuit applications of RTBTs are discussed. The chapter shows that RTBTs allow the implementation of many analog and digital circuit functions with a greatly reduced number of transistors and show considerable promise for multiple-valued logic. Experimental results on frequency multipliers and parity-bit generators are presented and analog-to-digital converters and memory circuits are discussed. The chapter concludes with a discussion on the effect of field-induced localization on vertical and parallel transport in quantum well structures.