Since its invention at Bell Labs in the mid-nineties, the quantum cascade laser (QCL) has evolved rapidly to become a viable commercial alternative to other solid-state mid-infrared light sources. QCLs are compact, high-power, wavelength-agile laser devices that are ideally suited for mid-infrared applications such as chemical sensing and free-space telecommunications. This paper summarizes our recent progress in QCL technology, with a particular emphasis on device design and fabrication. The current state-oftheart in performance of InP lattice-matched QCL is reviewed. In order to expand the functionality of QCLs, new materials and device concepts are being explored. In particular, nonlinear light generation in QCLs is discussed in detail. The experimental results confirm the potential of nonlinear QCLs to extend the wavelength range of InP-based devices. Ongoing efforts to improve the fabrication of InP-based QCLs and to develop new QCL materials are presented. © 2005 Lucent Technologies Inc.