An understanding of nanocrystal growth mechanisms is of significant importance for the design of novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years, there has been an increasing interest in liquid cell TEM and its applications include real time imaging of nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles (mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are provided.