The 2002 effusive flank eruption at Stromboli volcano started on December 28, after several months of strong explosive activity at the summit craters. On December 30, the seismic network recorded two large flank failures and associated tsunami waves. This is the first time that a flank collapse and tsunami, and their associated phenomena, have been recorded by a multi-disciplinary monitoring system. Volcanological and geophysical monitoring, as well as thermal surveys performed immediately before and after the failure, allowed us to define and interpret the sequence of events. The still on-going eruption has provided, for the first time, the opportunity to look into the dynamics of Stromboli's effusive eruptions, flank failure and landslide formation, and their potential hazard.