Over 16 years have passed since the Bell Labs group headed by Federico Capasso first demonstrated the quantum cascade laser (QCL). The intervening interval has seen the QCL evolve from a barely functional scientific curiosity into a powerful technology poised to offer new capabilities to a broad spectrum of real-world applications. In assembling this special section of Optical Engineering, the dual goals have been to provide a snapshot of the cascade laser field in its adolescence, and to familiarise a wider audience of optical engineers with the far-reaching practical potential of these unique IR sources.