La Réunion Island and Grande Comore are oceanic islands entirely formed by young and still active volcanoes, namely Piton de la Fournaise and Karthala. These geological features are responsible for a particular type of hydrosystem and present a unique challenge regarding the prospection and management of water resources. Young volcanic terrains are composed of lava flows with exceptional hydraulic conductivities. On both islands, despite high levels of precipitation, surface water resources are scarce due to high infiltration rates. Water infiltrates deep into the ground and reaches the water table at an elevation close to sea level. Access to water resources is particularly challenging on these islands due to: the relative absence of perennial surface water resources, the widespread marine contamination in littoral areas, the excessive depth of the groundwater table due to high elevations. We present the main results obtained from twenty years of multidisciplinary investigation programs dedicated to groundwater resources development in La Réunion Island. This approach includes geological and hydrogeological investigations, geophysical prospections and geochemical analysis. Results have led us to propose an improved conceptual model describing groundwater resources and to implement a numerical model of groundwater flow applied to Piton de la Fournaise volcano. In the interior of the island, the presence of a piezometric dome reaching a height of over 1200 m above sea level has been revealed and is of major interest for groundwater development plans.