  Self-Organized Criticality is the emergence of long-ranged spatio-temporal
correlations in non-equilibrium steady states of slowly driven systems without
fine tuning of any control parameter. Sandpiles were proposed as prototypical
examples of self-organized criticality. However, only some of the laboratory
experiments looking for the evidence of criticality in sandpiles have reported
a positive outcome. On the other hand a large number of theoretical models have
been constructed that do show the existence of such a critical state. We
discuss here some of the theoretical models as well as some experiments.
