Book excerpt: Achieving precise control over nanostructure evolution during synthesis is essential for creating next-generation nanomaterials with individually tailored properties. The first step in finely controlling the synthesis process of a nanostructure is to observe, measure and understand its growth mechanisms in detail, from the initial stages to the final product. However, due to limitations in the characterization techniques for liquid-solid systems, the prevalent approach to developing new nanostructures is largely a trial and error process of iteratively altering synthesis procedures and then characterizing the resulting nanostructures. This is fundamentally limited in that the growth processes that occur during synthesis can only be inferred from the final synthetic structure. Alternatively, directly observing real-time nanostructure growth within the synthesis environment could provide unprecedented insight into the relationship between synthesis conditions and product evolution, and facilitate a more rapid and systematic approach to nanostructure development.In this dissertation, the recently developed in situ liquid stage is used in the (scanning) transmission electron microscope ((S)TEM) to observe the real-time growth of mesoporous Pd nanostructures during synthesis within several organic micelle templates. The in situ (S)TEM results are used to identify Pd growth mechanisms that contribute to the disorder of the final pore structure, and to suggest possible ways of augmenting the template in order to improve its efficacy in directing the growth of ordered nanostructures. By observing in situ growth for various synthesis conditions, a refined synthesis procedure, which yields ordered pore structures, is ultimately determined. This ability to observe dynamic nanoscale liquid-solid systems in situ is essential for not only understanding the growth mechanisms during nanostructure synthesis, but also for understanding the processes that occur during the electrochemical operation of Li-ion batteries, such as electrode lithiation/delithiation and the formation of the solid electrolyte interphase (SEI), which strongly correlate with battery performance. By expanding upon the commercially available in situ liquid (S)TEM stage, a novel in operando nanobattery platform ("closed-cell") is developed to study the behavior of Li-ion battery electrodes and battery-relevant liquid electrolytes during electrochemical biasing. The structural evolution of a single Si nanowire, fully immersed in electrolyte, is observed in operando during electrochemical charging and discharging with controlled battery operating conditions, demonstrating the proof of concept and the unique capacity for fundamental battery research using this platform.