Sandpile models of self-organized criticality

Self-organized criticality is the emergence of long-ranged spatio-temporal correlations in non-equilibrium steady states of slowly driven systems with out fine tuning of any control parameter, Sandpiles were proposed as protot ypical examples of self-organized criticality. However, only some of the la boratory experiments looking for the evidence of criticality in sandpiles h ave reported a positive outcome. On the other hand, a large number of theor etical models have been constructed that do show the existence of such a cr itical state. We discuss here some of the theoretical models as well as som e experiments.