Film-based liquid cell transmission electron microscopy (TEM) has opened a new frontier in direct, atomic-scale visualization of nanocrystals within liquid environments. Ultrathin and flexible films supported on TEM grids, generally free from microfabrication, function simultaneously as electron-transparent windows and liquid encapsulation layers, enabling sub-nanometer spatial resolution for dynamic imaging of processes in solution. This technique enables real-time tracking of nucleation, growth, and etching at atomic resolution, revealing fundamental mechanisms underlying nanoscale transformations. This review examines the evolution of liquid cell architectures, analyzes key considerations including electron beam–sample interactions, solid–liquid–gas interface dynamics, and resolution limits, while highlighting the unique capabilities and emerging applications of film-based liquid cells. Particular emphasis is placed on breakthrough discoveries in elucidating metal nanocrystal structural evolution and in establishing structure–dynamics–function relationships. Looking forward, advances in diverse membrane materials, multi-field integration, and high-throughput analytical methods are expected to further enhance the versatility of film-based liquid cell TEM. These developments will deepen mechanistic insights into nanoscale dynamics in realistic liquid-phase reactions and accelerate the predictive design of compositionally and structurally complex metallic nanomaterials.