Nanometer and subnanometer particles and films are becoming an essential and integral part of new technologies and inventions in different areas. Some of the most common areas include the microelectronic industry, magnetic recordings, photovoltaic applications, and optical coatings. Because of the ultrasmall size at atomic levels, the effect of quantum size becomes prominent, and the sensitivity of size is defined even by a difference of a single atom. Additionally, the effect is of utmost importance as the single-atom catalysts are far more advantageous than conventional catalysts as they tend to anchor easily because of their low coordination. Also, the presence of a single-atom catalyst in reactions creates efficient charge transfer as it forms a strong interaction with the support. Furthermore, catalysts in the subnanometer regime exhibit different electronic states and adsorption capabilities compared to traditional catalysts. Therefore, to fully appreciate the subnanometer catalysis reactions, it is essential to study the means of characterizing the prepared subnano catalysts, in order to characterize the materials in their as-synthesized form, to obtain a precise and accurate analysis; these are some of the fundamental requirements for achieving an optimum performance. The physical properties of many interesting materials for advanced technological usage are highly governed by the distribution and placement of atoms. Superior techniques such as high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy and infrared and X-ray absorption spectroscopic techniques provide electronic and geometric configurations and also reveal the transformation of the subnano catalysts on the support material. Modeling methods such as density functional theory can successfully predict the electronic structure and geometric configuration of the catalyst, which in turn influence the selectivity and activity of the catalyst. Thus, understanding the characterization techniques gives the ability to understand, identify, and measure the local environment of individual atoms and the interaction with the surface support, which will give fundamental knowledge and insights in the realms of nanoscience and technology, materials science, chemistry, and physics. Therefore, detection and enhanced measurement of individual atoms is inherently challenging and is a prerequisite to the development of new technology and better performing materials. In this chapter, we have discussed various important methods of characterization for characterizing subnanometer and atomic catalysts. © 2020 American Chemical Society.