Piton de la Fournaise and Karthala are both shield volcanoes in the southwest Indian Ocean. This publication summarizes the work done on these very active basaltic volcanoes. Piton de la Fournaise has a long history of scientific research and monitoring, encompassing much data collected during recent eruptions. It is certainly one of the most studied volcanoes in the world. The work presented in this monograph includes geological, geophysical, geochemical and petrological aspects, as well as studies on physical geography, natural hazards and the sociological and behavioural implications. The Karthala volcano may be less well known, but it serves as an interesting comparison to Piton de la Fournaise. Although situated close to the volcanoes of Hawaii, it differs from them by its more alkaline magmas and less frequent activity. It was also monitored for more than 25 years, producing extraordinary eruptions in recent years.