To apply high-quality hyperthermia treatment to tumors at deep locations in the head and neck (H&N), a site-specific phased-array applicator was designed, built, demonstrated, validated by phantom measurements, and clinically introduced. This paper critically reviews the first clinical experiences and demonstrates the pivotal role of hyperthermia treatment planning (HTP). Three patient cases are discussed and their treatment planning, treatment, interstitially measured temperatures and interrelationships are analyzed from a physics point of view. Treatments lasting 1h were shown to be well tolerated with no acute treatment-related toxicity observed, but mean temperatures need still to be improved on. It was found that simulated power absorption correlated well with measured temperatures illustrating the validity of the approach. This is the first data proving that focussed heating of tumors in the H&N is feasible. HTP has proved to be a valuable tool in treatment optimization.