Tunable infrared diode lasers fabricated from single crystals of Pb 1− x Sn x Te and PbS 1− y Se y have been used for a wide variety of high resolution spectroscopy measurements in the wavelength range from 4 to 11 μm. Typical laser threshold current densities range from 50 to 100 A/cm 2 at 4.2°K and from 1000 to 2000 A/cm 2 at 77°K. CW operation is readily obtained at 4.2°K for diode lasers of these alloys and was also observed for PbS at 110°K. The diode laser cw output is typically 10–100 μ watt with an incremental external quantum efficiency of about 1 × 10 -3 . In a few cases, output powers of several milliwatts have been observed. Diode lasers have been made from Pb 1− x Ge x Te and Pb 1− x Cd x S alloys which show promise of extending the emission range of tunable lead-salt lasers to shorter wavelengths. Tuning of the laser emission frequency has been achieved by varying the diode current and temperature and by applying a magnetic field or pressure to the diode. The tuning rates of individual modes were measured using a Fabry-Pérot interferometer and ranged from 10 to 1000 MHz/mA for current tuning and approximately 1 MHz/Gauss for magnetic field tuning. These diode lasers have been extensively used in high resolution molecular spectroscopy. Absorption measurements have been made in sulfur hexafluoride (SF 6 ), ammonia (NH 3 ), ethylene (C 2 H 4 ), carbon monoxide (CO), and sulfur dioxide (SO 2 ). Several other experiments have been performed such as the observation of lambda-doubling, Zeeman splitting and nuclear hyperfine structure of (NO) and the direct observation of lineshape and of the gain (or loss) of several vibration-rotation lines of a CO gas laser. This paper describes the details of the physical properties of lead salt diode lasers as well as the current and potential uses of these devices.