Transmission Electron Microscopy (TEM) is a widely used materials characterization method, where electron beam alternating the sample needs to be avoided generally. While in some cases, electron beam can be used to introduce reactions that are useful for the understanding of materials properties. For example, for the study of nucleation and growth of inorganic nanoparticles from a liquid precursor solution, electron beam can trigger the reactions while serving as a light source for imaging. Although the electron beam induced chemical reactions can be complex and often hard to elucidate, the clear trend of inorganic nanoparticle growth may provide useful information for understanding of the reactions. We explore a wide range of systems undergoing chemical reactions induced by electron beam. Control experiments that lead to better understanding of the reactions are also introduced. Phenomena that are system specific and those that are more general will be highlighted. Perspectives on the future development in this area will be provided.