  The original sandpile model of Bak, Tang and Wiesenfeld from 1987 has
inspired lots of consequent work and further ideas of how to describe the birth
of scale-invariant statistics in various systems and in particular models. In
this article the basic ingredients of self-organized criticality (SOC) are
overviewed. In line with the orginal arguments of BTW SOC is now known to be a
property of systems where dissipation and external drive maintain a delicate
balance. The qualitative and quantitive understanding of the SOC state and
deviations from it can thus be addressed, by mapping the typical cellular
automata exhibiting SOC to theories, that in fact describe critical extended
systems as such. Currently two such approaches are known, based on the
connections of SOC to variants of absorbing state phase transitions and the
physics of elastic manifolds or interfaces in random media. These are reviewed
and discussed. Finally, some open theoretical problems and experimental
suggestions are outlined.
