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The characterization of nanomaterials involved in dynamic processes are conventionally conducted using microscopy, spectroscopy and other physical/chemical methods through the pseudo-dynamic approach. In details, the dynamics processes are recorded by repeating or terminating the process multiple times. However, the above approach can lead to missing important transition information and inducing contamination for mechanistic studies. This motivates the efforts to develop real time characterization techniques which can probe the dynamic change of nanoparticles in their native operating environments. With the capability of probing structural change at the nanoscale, in situ transmission electron microscopy, has shown great potential in studies and applications of various processes. Targeting at conducting precise analysis, which has been limited by many uncertainties from electron beam effects and the miniaturized reaction cell used for TEM, the work presented herein pursues a quantitative characterization of a few electrochemical and biological processes through in situ liquid-phase transmission electron microscopy. In this work, the in situ transmission electron microscopy system is evaluated by comparing the in situ results with those from standard experiments to show its capabilities in studying dynamic processes. The in situ system is quantitatively calibrated to obtain the optimized observation conditions to avoid detectable electron beam interference, solution depletion and achieve sufficient resolution for analysis through micrometer thick liquid. These form the fundamentals for the in situ studies. Moreover, a comprehensive analysis protocol is established by incorporating multiple ex situ and post situ characterizations. Using this optimized in situ system, the mechanism of electrodeposition of gold on carbon electrode is studied. The in situ results allow quantitative analysis of the growth process. The prevailing diffusion limited three dimensional growth model is examined. A study of the effect of supporting electrolyte on the electrodeposition of palladium is also conducted. The self-limiting, surface diffusion and aggregation/recrystallization growth model is found to describe the early stage of growth, rather than the classical Volmer-Weber growth model. A further study is conducted on the structural evolution of palladium nanoparticles under electrochemical cycling. The mechanisms involved in this process, including electrodeposition, dissolution, hydrogen co-deposition and hydrogen desorption, are studied. The supporting electrolyte, HCl, is found to enhance the dissolution of deposited palladium clusters and induce movements and aggregation of the deposits during the hydrogen interaction process to form chain-like and irregular clusters, which provide direct experimental proof on the morphology formation of palladium with hydrogen involvement. Ultimately, the in situ technique is applied to the study of calcium phosphate biomineralization. Combined with multiple post situ characterization techniques, the study provides direct experimental evidence of the non-classical pre-nucleation and attachment growth of calcium phosphate structures. This demonstrates the potential of the in situ technique for studying the mechanisms involved in biological processes. / Dissertation / Doctor of Philosophy (PhD) / Nanostructured materials have been widely used in various fields. In situ transmission electron microscopy, a technique used to characterize nanomaterials involved in different dynamic processes in their operating environments, is an advanced tool over the traditional characterization methods such as ex situ microscopy and spectroscopy. However, there are several challenges in applying this in situ technique to processes occurring in liquid media. In this thesis, an in situ transmission electron microscopy system is applied to study the mechanisms of structural changes during different processes in liquids with both high spatial and temporal resolution. Protocols to evaluate and optimize the in situ system are developed to provide results comparable with those from their actual applications. Then in situ studies on the structural evolution of nanomaterials during electrochemical processes are performed and different theoretical models are applied to describe these processes. Finally, this technique is extended to investigate biomineralization to show its capabilities in future studies on biological processes.