Simulation studies can estimate the potentials of regional hyperthermia based on the multi-antenna principle. They postulate an optimal parameter setup (phases, amplitudes) that should increase temperatures in heat treatments by 1 degrees C or more. However in praxi, slight inaccuracies and uncertainties accumulate in a way that the optimal adjustment is typically missed during a real treatment. The reasons for these errors are electrical and geometrical. A way out is an on-line control, which adapts the plans to the actual distributions. Magnetic resonance thermography is employed for on-line control, and control software has been developed. However, there is still a long way to reach the technological endpoint of multi-antenna systems for clinical applications.