This paper provides an overview of the Social Event Detection (SED) task, which is organized as part of the MediaEval 2011 benchmarking activity. With the convergence between social networking and multimedia creation and distribution being experienced on a regular basis by hundreds of millions of people worldwide, this task examines how new or state of the art techniques can cope with the need for detecting social events by automatically analyzing the social multimedia content. This paper discusses the challenges set as part of the SED Task, the dataset that was provided to the task participants, and the process of evaluating the submissions.