  This is a somewhat expanded version of the notes of a series of lectures
given at Lausanne and Stellenbosch in 1998-99. They are intended to provide a
pedagogical introduction to the abelian sandpile model of self-organized
criticality, and its related models : the q=0 state Potts model, Takayasu
aggregation model, the voter model, spanning trees, Eulerian walkers model etc.
It provides an overview of the known results, and explains the equivalence of
these models. Some open questions are discussed in the concluding section.
