In this paper, we provide an overview of the Social Event Detection (SED) task that is part of the MediaEval Benchmark for Multimedia Evaluation 2013. This task requires participants to discover social events and organize the related media items in event-specic clusters within a collection of Web multimedia. Social events are events that are planned by people, attended by people and for which the social multimedia are also captured by people. We describe the challenges, datasets, and the evaluation methodology.