Microwave interstitial hyperthermia now plays an increasing part in the various hyperthermia techniques used in cancer treatment. This paper will present the design, construction and theoretical as well as experimental study of miniature coaxial antennas designed to heat tumours of various volumes and sizes. Monitoring temperature using multifrequency radiometry during hyperthermia stems naturally from the very design of these antennas. Experiments on phantoms, animals and patients in phase I clinical assessment have demonstrated that microwave interstitial hyperthermia can be achieved with temperature control and monitoring by microwave radiometry.