It is critical to understand how electrochemical materials change, transform, and degrade within devices to enable the development of next-generation energy storage and conversion systems. In my research group, multi-scale in situ techniques are used to reveal reaction mechanisms and interfacial transformations in materials for batteries and catalysis. In this talk, I will present our recent work on understanding and controlling transformations at interfaces between solid-state electrolytes and electrode materials within solid-state batteries, and how these interfacial transformations influence chemo-mechanical degradation. Next, I will discuss our work on investigating phase transformation pathways in high-capacity battery electrode materials using in situ transmission electron microscopy. In particular, unexpected chemo-mechanical stability is found during reaction of sulfide electrodes with larger alkali ions than Li+. Finally, results will be discussed related to growth mechanisms of layered chalcogenide materials in the presence of various transition metals; this work is important for designing catalysts with tailored structure and morphology. Overall, this research demonstrates how fundamental understanding of dynamic processes can be used to guide the design and engineering of new energy materials with improved lifetime.