Radical species play key roles in chemical vapor deposition (CVD) processes, including catalytic CVD (hot-wire CVD), and evaluating their densities in the gas phase is important in understanding the underlying mechanisms and controlling the processes. There are many diagnosis techniques to detect radical species. Among them, photon-in-photon-out techniques, including one-and two-photon laser-induced fluorescence and photo-absorption, can be utilized under the low vacuum conditions typical in CVD processes. Highly sensitive, quantitative, state-specific, real-time, and non-intrusive detection is possible with these techniques; even spatial resolution can be obtained in some cases. At the same time, however, we should know the drawbacks and the limitations of such techniques. A compilation of the radical species detected so far in catalytic CVD processes is presented in a table. (C) 2014 Elsevier B. V. All rights reserved.