  Since their introduction in the 80s, sandpile models have raised interest for
their simple definition and their surprising dynamical properties. In this
survey we focus on the computational complexity of the prediction problem,
namely, the complexity of knowing, given a finite configuration $c$ and a cell
$x$ in $c$, if cell $x$ will eventually become unstable. This is an attempt to
formalize the intuitive notion of "behavioral complexity" that one easily
observes in simulations. However, despite many efforts and nice results, the
original question remains open: how hard is it to predict the two-dimensional
sandpile model of Bak, Tang and Wiesenfeld?
