Single-mode and tunable quantum cascade distributed feedback (QC-DFB) lasers in the wavelength range from 4.5 to 16.5 mum are reviewed. In the case of QC lasers with dielectric waveguides, DFB lasers are fabricated either with a top-grating approach, which is simpler to manufacture, or a buried grating with epitaxial regrowth, which generally has a higher single-mode yield as a result of a larger coupling factor. Long-wavelength QC-DFB lasers based on surface plasmon waveguides use bi-metal gratings for Bragg reflection. Single-mode emission with a side-mode suppression ratio of 30 dB and a tunability (depending on wavelength) of 0.3-1.0 nm/K heat sink temperature or of 2040 nm/A CW current are customarily achieved. These features together with the potential for high optical power, room-temperature operation, and narrow intrinsic linewidth make QC-DFB lasers prime choices as narrow-band light sources in mid-infrared trace gas sensors. As a result of their unipolar nature and the possibility to serially stack, or "cascade," many active regions, QC lasers also have an intrinsic potential for multiple-wavelength operation in a wide variety of device concepts. Multiple different optical transitions in single active regions stacked in a homogeneous cascade as well as multiple single-wavelength active regions cascaded in various schemes of heterogeneous cascades have been demonstrated. Based upon multiple-wavelength QC lasers, multiple single-mode QC-DFB lasers have been fabricated using sectioned laser cavities with multiple gratings. Adjusting the length of each Bragg-grating section as well the mode-overlap factor by tailoring the heterogeneity of the cascade has lead to a doubly single-mode QC-DFB laser with simultaneous single-mode emission around 5.0 and 7.5 mum and a tunability at each wavelength as expected from equivalent single-wavelength single-mode lasers. Finally, the concept of multiple-wavelength emission was extended to ultrabroad-band emission, with a QC laser that exhibited gain from 5 to 8 mum and simultaneous laser action from 6 to 8 Am.
