Materials synthesis and the functioning of devices often involve liquid media.
However, direct visualization of dynamic processes in liquids, especially with
high spatial and temporal resolution, has been challenging. For solid materials,
advances in aberration-corrected electron microscopy have made observations of
atomic-level features a routine practice. Here, we discuss the extent to which
one can take advantage of the resolution of modern electron microscopes to image
phenomena occurring in liquids. We describe the fundamentals of two different
experimental approaches that use closed and open liquid cells. We illustrate the
capabilities of each approach by considering processes in batteries and
nucleation and growth of nanoparticles from solution. Liquid-cell electron
microscopy appears to be duly fulfilling its role and promise for in
situ studies of nanoscale processes in liquids, revealing physical
and chemical processes that are otherwise difficult to observe.