Air pollution monitoring with tunable lasers is gaining widespread acceptance. The lasers offer unique capabilities related to remote sensing and in situ monitoring; the systems are generally simple and straightforward to calibrate. Tunable lasers for monitoring such as dye lasers, optical parametric oscillators (OPO), semiconductor diode lasers, spin-flip-Raman (SFR) lasers, and high-pressure gas (HPG) lasers are discussed and compared. Proposed applications for the following four different monitoring schemes that use the differential absorption technique are presented: active bistatic system involving a cooperative reflector or remote detector; active monostatic system involving a natural or topographic reflector; active monostatic system involving aerosol backscattering (this may also be bistatic if the detector is situated away from the laser transmitter); passive monostatic system involving heterodyne detection. The term ''bistatic'' refers to an arrangement in which some of the equipment is positioned at a distance from the laser, while in a ''monostatic'' system, all equipment is at a single location. As yet, there are no commercial tunable laser systems designed specifically for any of the pollution monitoring applications described. Historically, progress in this field has been largely instrument-limited: lasers with limited power, tunability, stability; signal detection techniques with inadequate sensitivity or speed; computers with limited more » capabilities or field reliability. Improvements continue to be made in all of these areas in laboratories of many countries. It appears certain, therefore, that laser monitoring of pollutants in the troposphere and stratosphere will become increasingly important for our understanding and surveillance of the atmosphere. « less