Sustainable technologies will be essential for meeting future energy demands. Therefore, it is critical to be exploring the fundamental science of materials for photovoltaics and catalysis. For these applications, nanomaterials hold great promise due to their unique, size-dependent properties, tunability, and solution processability. Additionally, nanomaterials offer an effective testbed to understand structural transformations in bulk materials, and to take advantage of effects related to having a high surface area to volume ratio. Transmission electron microscopy (TEM) allows a unique window into the atomic structures of nanomaterials by enabling high-resolution imaging, electron diffraction, and compositional analysis. Specifically, this work explores local structural and compositional transformations by examining how colloidal nanocrystals transform under in situ conditions during imaging. Focusing on halide perovskite nanocrystals for photovoltaic applications and metal nanocrystals for catalytic applications, the roles of size, shape, and composition will be explored, as well as the impact of atomic defects. The degradation of halide perovskites is a major issue facing the field and can be mitigated by an in-depth understanding of the structural changes that occur during degradation processes. For instance, the relative stability of different compositions and dissimilar geometries can provide insights into which factors could improve the longevity of photovoltaic devices. Similarly, metallic nanocrystals for heterogeneous catalysis rely on reactive surfaces, which can be identified, examined, and transformed during in situ TEM experiments. This presentation will describe progress toward understanding transformation mechanisms in these two classes of nanocrystals, and it will provide comparative insights across the systems.