We have recently developed a radical measurement method using infrared diode laser absorption spectroscopy (IRLAS) and succeeded in measuring the SiH₃ radical density in DC pulsed and RF silane plasmas, and the diffusion coefficient and reaction rate constant of the SiH₃ radical for the first time. The correlation of the SiH₃ radical density and a-Si : H deposition rate in DC pulsed SiH₄/H₂ plasma was also measured. Although this SiH₃ radical has been assumed to be the most important precursor of a-Si:H thin film formation, it has never been measured. This IRLAS can be applied also to SiH and SiH₂ radicals in silane plasma, the CF radical in CF₄/H₂ plasma, the CH₃ radical in methane plasma, etc. It has been demonstrated by studies mentioned above that IRLAS is a very widely-used spectroscopic method which is applicable to the measurement of various radicals in many processing plasmas. In this review, IRLAS and typical radical measurements by it are described.