Abstract: Knowledge of the ionizing radiation induced chemical reactions has been Continued from p. 16) sufficiently found for radiation chemistry which was regarded as a distinct scientific discipline since 1940s.This article aims to review the background and fundamentals concerning the initial consequences of interaction of radiation with matter and the kinetics governing the primary species produced.The first part of the article emphasizes the "primary products" ,the initial species created by energy absorption, which are excited molecules and ions and electrons with energy greater than thermal and attempts to cover the state of this research to date.