Progress on the modeling, fabrication, and characterization of the transistor-injected quantum-cascade laser (TI-QCL) is presented. As a novel variant of the quantum cascade laser, the TI-QCL has been projected to have advantages over conventional QCLs in certain applications because of its 3-terminal nature. The separation of field and current is expected to allow separate amplitude and frequency modulation, and the location of the cascade structure in a p-n junction depletion region is expected to reduce free carrier absorption and improve efficiency. At the same time, the added complexity of the structure creates challenges in the realization of working devices. An overview of the basic operating principles of the TI-QCL is first given, and projected advantages discussed. Next, work on modeling GaAsbased TI-QCLs is presented, and a design for devices in this system is presented. Finally, work on fabrication and characterization of devices is examined and ongoing challenges are discussed. The role of quantum state alignment in the QCL region on electron-hole recombination in the base is also examined, showing the capability of using basecollector voltage to modulate the optical output from the direct-bandgap transistor base.