There are many strong fundamental ro-vibrational molecular transitions in the mid-IR (2.5-10 μm), making this an extremely useful region of the electromagnetic spectrum for trace gas analysis in environmental or medical monitoring applications. In general, the fundamental vibrational bands active in this region have stronger line strengths than the overtone and combination bands typically used in the visible and near-IR regions. In addition, spectra are less congested, allowing selective spectroscopic detection of a large number of molecules. The 3 μm region is of particular importance due to the strong vibrational bands associated with the C-H, N-H, and O-H stretches. Two major ways of generating mid-IR laser light are quantum cascade lasers (QCLs) and difference frequency generation (DFG). These sources offer excellent versatility and sensitivity for many medical applications.