Placing metals within microwave ovens has been generally viewed as a dangerous practice because of occurrence of violent discharge, but in recent years such discharge phenomenon has attracted increasing attention and has enabled a variety of exciting applications. In this work, we provide a comprehensive review of fundamental understanding of microwave–metal discharge interaction and its state-of-the-art application for nanomaterials synthesis. We introduce the microscopic interaction between different categories of materials and the electric and magnetic field of microwaves. For microwave–metal interaction, we highlight its size-dependence and point out the influence of the oxide layer on the surface of metals. We discuss the required conditions for occurrence of discharge, microscopic formation mechanism, and characteristic features of microwave–metal discharge processes. Through analyzing the influence from the microwave input, discharging metals, and surrounding discharging media, we discuss the strategy for systematical regulation of the discharge process. We describe the applications of the microwave–metal discharge for facile synthesis of various functional nanomaterials including core–shell carbon/metal, metal oxides, metal chalcogenides, intermetallic compounds, metallic nanoparticles and metallic compounds, and organic compounds. Finally, the challenges in precise characterization and dynamic regulation of the discharge process as well as exciting application opportunities are discussed.