Building functional nanostructures with atomic level precision requires a detailed understanding of materials growth, and in particular the physics of self-assembly at the nanoscale. Experiments in which a self-assembly process takes place in situ in the transmission electron microscope allow continuous observations of individual nanostructures, and even measurements of their properties, while they grow and change. The resulting movies and measurements help to quantify the mechanisms that control nanostructure formation and can also reveal unexpected reaction pathways. I will describe experimental techniques to probe processes taking place in both vapour and liquid phase. I will illustrate with examples including nanocrystal epitaxy on graphene and electrochemical deposition of metals. I will discuss how new types of nanostructure can be designed using the self-assembly phenomena revealed in situ and consider some possible applications. Finally I will provide a perspective on the exciting recent advances in transmission electron microscopy that will impact in situ growth experiments in the future.
