Scanning transmission electron microscopy (STEM) is a key tool in the advancement of materials science. As nanoscale materials and atomically sharp interfaces become increasingly technologically relevant, STEM provides feedback on the individual atoms, defects and interfaces that matter in the material. Two areas where STEM characterization is critical for materials advancement are electrochemical energy materials, such as battery electrodes and fuel cell electrocatalysts, and complex oxide materials, which exhibit a diverse array of properties. Electrochemical energy systems, including battery electrodes and electrocatalysts for fuel cells, are critical for the future of clean transportation. For these materials, investigation of the nanoscale processes which occur in liquid electrolytes is critical for understanding their performance in real devices. Encapsulating a thin layer of liquid in the TEM for in situ characterization is an exciting approach for gaining a detailed understanding of the underlying mechanisms of energy conversion and storage. The first half of this dissertation discusses the development and use of liquid-cell and electrochemical-cell TEM. First, I discuss the limitations and opportunities of EELS in thick liquids. I further discuss an electrochemical cell for the TEM, and its application to fuel cell electrocatalysts and battery materials. I was able to observe Pt-Ni octahedral catalyst particles degrade and lose their shape, and detect lithium ions move in a charging and discharging battery cathode (LiFePO4) with nanoscale detail by mapping the electronic structure. The second part of the dissertation focuses on complex oxides, which host a wide range of electronic and magnetic properties. Oxide interfaces, such as thin-film heterostructures and domain walls, are often atomically abrupt and produce novel functionalities. In particular, ferroelectric domain walls are rich sources of emergent phenomena – such as two-dimensional conductive sheets that form in an otherwise insulating solid – due to their unusual electronic properties or symmetry breaking. We probed how ferroelectric distortions change at domain walls with STEM for ErMnO3. From the STEM images, we calculated the order parameter at many domain walls in ErMnO3 to create a statistical picture of the ferroelectricity at these topological defects, and also observed how the order parameter changes near vortex structures. We further apply these methods to lutetium ferrite superlattices, where we use feedback from STEM to design a near-room-temperature multiferroic material in which ferroelectricity enhances the magnetism. We find that there are confined charged domain walls in the superlattice that additionally boost magnetism.