Chemical sensing has enormous potential in environmental monitoring, medical applications and industrial process control. Recent studies have linked airborne heavy carbon isotopes to the early formation of ulcers and some stomach cancers. New regulations calling for safer work environments and pollution control require tighter control of pollutant concentrations. Industrial processes require accurate control of reactants or contaminants to lower waste production and raise process efficiencies. The ability to detect chemical species with high sensitivity and reliability is an ever-increasing necessity for our society. Compared with other sensing methods, optical sensing combines two important requirements for system applications: sensitivity and fast response. Optical sensing also can measure the concentration of substances that are remote from an instrument. This is the case for both lidar and chemical analyses performed through a window on samples within a reaction vessel. Overall, the market for sensing applications amounts to more than $1 billion. However, it has been marginally accessible to optical systems because of their complexity, cost and maintenance. The commercialization of quantum cascade lasers could significantly reduce the price of optical sensing and open opportunities for a wider market.