  We present a pedagogical introduction to self-organized criticality (SOC),
unraveling its connections with nonequilibrium phase transitions. There are
several paths from a conventional critical point to SOC. They begin with an
absorbing-state phase transition (directed percolation is a familiar example),
and impose supervision or driving on the system; two commonly used methods are
extremal dynamics, and driving at a rate approaching zero. We illustrate this
in sandpiles, where SOC is a consequence of slow driving in a system exhibiting
an absorbing-state phase transition with a conserved density. Other paths to
SOC, in driven interfaces, the Bak-Sneppen model, and self-organized directed
percolation, are also examined. We review the status of experimental
realizations of SOC in light of these observations.
