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Understanding the microscopic mechanisms of electrochemical reaction and material degradation is crucial for the rational design of high‐performance lithium ion batteries (LIBs). A novel nanobattery assembly and testing platform inside a transmission electron microscope (TEM) has been designed, which allows a direct study of the structural evolution of individual nanowire or nanoparticle electrodes with near‐atomic resolution in real time. In this review, recent progresses in the study of several important anode materials are summarized. The consistency between in situ and ex situ results is shown, thereby validating the new in situ testing paradigm. Comparisons between a variety of nanostructures lead to the conclusion that electrochemical reaction and mechanical degradation are material specific, size dependent, and geometrically and compositionally sensitive. For example, a highly anisotropic lithiation in Si …