Room-temperature, near-infrared, semiconductor diode lasers continue to gain importance for gas monitoring applications owing to their compactness, ease of use, reasonable cost and compatibility with telecommunications-grade optical fiber components. They may probe overtone or combination vibrational bands for a large variety of atmospheric relevant molecular species. These spectral bands exhibit line strengths orders of magnitude lower than those of fundamental vibrations, occurring in the mid-infrared. As a consequence, they are often used in conjunction with long-path techniques, enabling one to perform high sensitivity local measurements through long absorption path-lengths. At this purpose, resonant optical cavities can be fruitfully employed. This paper is devoted to a discussion of the main features of cavity-enhanced absorption spectrometers, operating with near-infrared diode lasers. We report on the operating principle as well as the achievable performance of these devices, also compared to more traditional apparatus, based on the multiple reflection cells. Experimental results on water vapour and oxygen detection are reported.