Laser based gas detection and monitoring techniques have now evolved to a mature level. Critical laser performance parameters include spatial beam quality, usable IR power, linear frequency tunability and stability. For continuous-wave, long-path absorption spectroscopy, the development of robust mid-infrared spectroscopic sources has led to numerous selective, sensitive and real-time gas monitoring applications. These new compact and tunable spectroscopic sources (<0.5 cubic feet) can be designed for efficient room-temperature operation in the 2.4 - 4.6 microns wavelength region using standard near-IR telecom lasers that are optically mixed in nonlinear optical materials such as periodically poled LiNbO3 (PPLN). Wavelength multiplexing and flexible dispersion control of PPLN crystals offer convenient narrow-linewidth (100 kHz - 2 MHz), single or multiple-frequency mid-IR operation at the milli-watt level. This permits the sensitive detection of many molecules such as HF, HCl, CH2O, CH4, CO2, CO and N2O at their strong fundamental rotational-vibrational transitions using direct, dual-beam, 2-f and other advanced spectroscopic detection schemes. At this wavelength region, these new laser sources provide an ideal alternative to cryogenically cooled lead-salt diode lasers. This paper will focus on the comparison of the two technologies with an emphasis on achieving ultra-high sensitivity in ground and airborne applications.