Book excerpt: Active-site is the region at the central atom’s position where functional activated reactions occur in many materials. Hence, it is important for the present study of material sciences to take into consideration this information of atomic structures in the reaction center of the localized impurities and catalyst and phase boundary and the photosynthetic reaction centers. However, it is very difficult to determine a three-dimensional atomic structure directly in the center positions of many functional materials.This book is written for readers to gain the basic knowledge of this “active-site”. It will benefit those who want to know the function and structure of the inorganic, organic and biological materials.