Various state-of-the-art optical techniques used for trace gas sensing are presented. Hollow-core fiber (HCF) based absorption spectroscopy and Photoacoustic Spectroscopy (PAS) low-volume measurement techniques are demonstrated for ammonia (NH3), methane (CH4), carbon dioxide (CO2), carbon monoxide (CO) and acetone (C3H6O), which are important for environmental, other industrial monitoring, and biomedical monitoring applications. A supercontinuum laser (SCL) source (from 0.9 to 2.5 μm) and a quantum cascade laser (QCL) source (from 7.9 to 8.4 μm) are used. The HCF-based measurements using a 10 m long fiber measured concentrations of NH3, CH4, CO2 and C3H6O in the order of 13, 15, 154 and 30 parts-per-million (ppm), respectively. While, in the PAS, the detection limits in sub-ppm levels for NH3 as 0.36 ppm, CH4 as 0.48 ppm and CO as 2.59 ppm are attained. The interference of various species in the gas mixture is eliminated by adopting the dual-wavelength measurement approach.