Three lectures on 3-algebras

These notes are based on lectures given in Valencia in October 2008 and in Stockholm in November 2008, in the framework of the Nordita workshop "Geometrical aspects of String Theory". We introduce the notion of a metric 3-Lie algebra and review some of the classification results. We explain the deconstruction of metric 3-Lie algebras in Lie algebraic terms and introduce a general framework in which to describe other 3-algebras of relevance in the description of three-dimensional superconformal Chern-Simons theories, particularly those with N=6. The emphasis throughout is on the general ideas and concrete examples.