Much work has been done on Karthala volcano over the last 30 years concerning the volcanology, structural geology, petrology, geochemistry and geophysics. We focus here on the current state of knowledge about the structure of this typical shield volcano and its recent eruptive activity. Karthala is a large basaltic shield volcano with a typical “Hawaiian” shape, with two well-developed rift zones diverging from a polylobate summit caldera complex. Karthala has not been dissected by erosion, although the concave morphology of its southern and eastern flanks may have resulted from huge flank landslides. Karthala volcano is one of the most active African volcanoes. During the last two centuries, most of the eruptions have been magmatic, emitting alkali basalts. They have been characterised by eruptive fissures which opened along the rift zones, within the caldera, or at low elevation far from the rift zones and the caldera. Karthala’s eruptive style is mostly effusive; however, phreatic and phreatomagmatic explosions also occur. Karthala erupted regularly (with an average frequency of one eruption every 6–8 years) over the past 100 years until the phreatic eruption of 1991. Since then, its activity has increased, with four eruptions from 2005 to 2007, two of them with significant impacts on the local population. The two phreatomagmatic to violent-strombolian eruptions in 2005 were more explosive and long-lasting than the preceding eruptions, projecting ashes and volcanic debris onto the eastern part of the island, affecting as many as 245,000 people. In the recent past, the 1977 eruption was the most destructive of Karthala’s historic lava flows. But in 1858, a lava flow travelled 13 km from the upper part of the north rift zone to the western coast, going close to the capital Moroni. Wherever the location of the eruptive vent, lava flows can reach inhabited areas and the sea in a few hours.