The environmental transmission electron microscopy (E-TEM) is a budding technique for in situ study of gas–solid chemical reactions with numerous applications. Recent improvements in the design have made it possible not only to obtain atomic level information but also the chemical information during the reaction by incorporating an imaging filter or electron energy-loss spectrometer to an E-TEM. We have been involved in modifying a couple of microscopes to incorporate environmental cells in order to convert them into E-TEMs. These microscopes have been used to obtain atomic level information of the structural and chemical changes during dynamic processes by in situ electron diffraction, high-resolution imaging, and electron energyloss spectroscopy. The applications include, but are not limited to, oxidation, reduction, polymerization, nitridation, dehydroxylation, hydroxylation, chemical vapor deposition, etc. We report recent developments in the design and application along with the limitations of an E-TEM.