The detailed description, the experimental demonstration, and the design of single‐mode microwave cavity applicators are presented. These microwave applicators utilize an internally tuned, single‐mode (or selective multimode) cylindrical cavity to focus and match microwave energy into cylindrical rod, disk‐, or slab‐shaped material loads. The combination of mode focus control and variable, internal cavity matching allows the efficient coupling of microwave energy into a wide range of solid material loads. The experimental operation of these applicators is demonstrated for low loss to lossy materials, which include nylon, water, and semiconducting silicon wafers. Overall microwave coupling efficiencies of 70%–95% were measured and experiments demonstrated the ability to precisely control material heating. The single‐mode fields can be used to heat and diagnose during processing, and on‐line process diagnosis was experimentally demonstrated using cavity perturbation theory. When using these cavity applicators, many system variables such as tuning control, cavity electric field strength, input, and reflected power are available as potential process system feedback signals for automated intelligent processing. These applicators can be used to carefully study the fundamentals of electromagnetic interactions of many materials. Applications include the electromagnetic heating of foods, semiconducting and biological materials, and the heating and curing of epoxy and composite materials.