The applications of near-IR spectroscopy with DFB lasers and fibre optic networks are reviewed. Since near-IR absorption lines are relatively weak, techniques to enhance the sensitivity are reviewed, including the use of multi-pass cells, ring-down spectroscopy and the various forms of cavity-enhanced spectroscopy. Examples of non-enhanced gas cellsconsidered include micro-optic cells for integration with optical fibre networks, evanescent-wave cells on silicon chips, and open-path free space propagation for atmospheric monitoring based on collection of scattered light or from a retro-reflector. The design of fibre optic sensor networks is discussed in detail particularly the use of spatial-division multiplexing for multi-point detection of gases over large areas. Throughout the chapter, a number of application areas are considered with examples given of near-IR systems employed for the detection of gas leaks from pipelines and storage facilities, characterisation of combustion processes, tomographic imaging of carbon dioxide in aero-engine exhaust emissions, imaging of hydrocarbons within internal combustion engines and atmospheric sensing of water vapour and greenhouse gases.