De Heer and his research groups have made significant contributions to several important areas in nanoscopic physics. As a graduate student at UC-Berkeley, he participated in groundbreaking research on alkali metal clusters that demonstrated the electronic shell structure of metal clusters.This is a property of small metal clusters composed of few atoms that develop atom-like electronic properties (these clusters are also referred to as superatoms). In Switzerland, he developed methods of measuring the magnetic properties of cold metal clusters and described how magnetism develops in these clusters as their size increases from atomic to bulk.He is the author of the most highly cited review articles on metallic clusters.