This talk will focus on recent advances in the development of sensors based on quantum cascade (QC) and interband cascade (IC) lasers for the detection, quantification and monitoring of trace gas species and their application to environmental monitoring, industrial process control and medical diagnostics. The development of compact trace gas sensors based on QCLs and ICLs will permit the targeting of strong fundamental rotational-vibrational transitions in the mid-infrared. The architecture and performance of several sensitive, selective and real-time gas sensors operating in the mid-infrared will be described. To date we have detected 14 gases (CH4, N2O, CO2, CO, NO, H2O, SO2, NH3, C2H 4, OCS, C2H4, H2CO, C 2H5OH and C2HF5) at the ppm to ppt level. In several cases, isotopic signatures of carbon and oxygen have also been observed.