Glass ceramics, i.e. intentionally devitrified glasses, are used in manifold applications. This is due to the capability to precisely control the precipitation of crystalline phases possessing useful properties related to optics, mechanics, biocompatibility, piezoelectricity, machinability, etc. In order to control the nano and microstructure, transmission electron microscope imaging proves particularly useful. Illustrated by numerous examples, the diverse precipitate shapes and mechanisms at work upon crystallization of glasses are discussed. In addition, it is shown how useful cutting-edge transmission electron microscopy is in the field of nanoanalytics. The investigation of nanoscale processes controlling the precipitation of functional crystals evidences the need for more sophisticated theories describing the changes of composition accompanying crystallization in non-isochemical systems (precipitates possess compositions different from those of the host glasses).