The fabrication of nanostructures can be greatly assisted by the use of in situ tools. Reflection high-energy electron diffraction (RHEED) is a classic tool for in situ monitoring of growth by molecular-beam epitaxy, and it has gained renewed relevance for the production of quantum dots. The evolution of RHEED spot intensities provides information about quantum dot ensembles during nucleation, growth, growth interruptions, decay, and dissolution. The integrated RHEED spot intensity evolves in a characteristic way whether InAs or InxGa1-xAs is grown on GaAs(100), and whether quantum dots are growing or decaying. The application of RHEED to practical issues in the growth of quantum dots is described.