High intensity ultrasound was demonstrated to provide an effective and minimally invasive therapeutic tool for in situ ablation of localized cancers. MRI guidance of the treatment allows excellent volume targeting, together with accurate on line measurement of the actual temperature in tissues. Spatial and temporal automatic feedback control of the procedure can guarantee effectiveness and safety against tissue and individual biovariability of physical parameters. This paper provides a general overview of authors' experience on the active control of the therapy with MR-compatible applicators of extracorporeal and endocavitary high intensity ultrasound.