The properties of materials are closely linked to their microstructures, making microstructural analysis essential in materials research and development. Transmission electron microscopy (TEM) is a powerful technique that enables the observation and analysis of material structures and behavior at the atomic level. However, because TEM uses an electron beam, specimens must be placed in a vacuum, meaning that observations are made under conditions far removed from the environments in which materials are actually used. This has created a demand for in situ TEM techniques, which allow observation under conditions close to real operating environments. Catalysts, for example, which are attracting attention as a solution to environmental and energy problems, are often used in gas atmospheres at high temperatures. To enable in situ observation of such catalytic materials, our group has developed an in situ observation system based on TEM specimen holders. Because the specimen holder is independent of a microscope itself, it can be flexibly developed and modified to suit the target of observation. Furthermore, recent advances in microfabrication technology have made it possible to integrate multiple components into the holder tip, only a few mm thick, to create a variety of environmental conditions. By combining this system with various TEM observation and analysis techniques, we can observe and measure catalysts from multiple perspectives and capture changes in their microstructure and composition under operating conditions. Through this approach, we aim to elucidate the reaction and degradation mechanisms that hold the key to improving the efficiency, durability, and cost-effectiveness of catalysts.