The unprecedented control of ultra‐thin‐film properties required by advanced device concepts has led to the development of novel techniques for both growth and characterization, including cathodoluminescence microscopy, scanning tunneling microscopy and in situ optical diagnostics. In situ electron microscopy can be used to directly observe transient growth processes, revealing unexpected features such as the lateral growth of Ga layers around droplets when the surrounding surface is covered with As (Figure). magnified image magnified image