Energy devices such as rechargeable batteries, fuel cells, and solar cells are central to powering a renewable, mobile, and electrified future. To advance these devices requires a fundamental understanding of the complex chemical reactions, material transformations, and charge flow that are associated with energy conversion processes. Analytical in situ transmission electron microscopy (TEM) offers a powerful tool for directly visualizing these complex processes at the atomic scale in real time and in operando. Recent advancements in energy materials and devices that have been enabled by in situ TEM are reviewed. First, the evolutionary development of TEM nanocells from the open-cell configuration to the closed-cell, and finally the full-cell, is reviewed. Next, in situ TEM studies of rechargeable ion batteries in a practical operation environment are explored, followed by applications of in situ TEM for direct observation of electrocatalyst formation, evolution, and degradation in proton-exchange membrane fuel cells, and fundamental investigations of new energy materials such as perovskites for solar cells. Finally, recent advances in the use of environmental TEM and cryogenic electron microscopy in probing clean-energy materials are presented and emerging opportunities and challenges in in situ TEM research of energy materials and devices are discussed.