In order to realize the high performance of material process, various measurement methods for radical densities in the reactive plasmas have been developed and the behaviors of important radicals in the gas phase, which have been impossible to measure in the past, have recently been clarified. With these measurement techniques, it is requested strongly that the quantitatively understanding of kinetics of the surface reaction of radicals and hereby clarifying the precursors for etching and thin film deposition for controlling the process with a high accuracy. In this article, the findings of measurement techniques of radical densities, the behaviors of radicals in the gas phase and on the surface have been described. (C) 2002 Elsevier Science B.V. All rights reserved.