This proposal is designed to combine high-performance Quantum Cascade Lasers with the most advanced cavity-enhanced spectroscopic methods in order to develop new trace-gas methods. This project will lead to a non-invasive and compact, diagnostic tool for the real-time analysis of trace gas concentrations in human breath. The detection in human breath of gases with medical relevance, and particularly of hydrogen peroxide (H2O2), and hydrogen cyanide (HCN), will be our main challenge.The first innovative step of this research will be the development of a continuous wave mid-infrared External Cavity Quantum Cascade Lasers (EC-QCL), offering broad frequency tuning range in the 4-11 micrometer infrared region, high tuning speed, and high output power. This laser source will be able to probe a vast majority of molecular gases exhibiting strong or fundamental vibrational absorption bands in the mid-infrared.For this project the detection sensitivity has to be pushed to extreme limits. Therefore, as a second step, Frequency Modulation of the EC-QCL source will be developed, to perform for the first time with QCLs, Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS). Based on the use of high-finesse optical cavities, it will enable the tracing of molecules at the low part-per-trillion volume: the required sensitivity to trace exhaled biomarkers from humans.