  We discuss the properties of a self--organized critical forest--fire model
which has been introduced recently. We derive scaling laws and define critical
exponents. The values of these critical exponents are determined by computer
simulations in 1 to 8 dimensions. The simulations suggest a critical dimension
$d_c=6$ above which the critical exponents assume their mean--field values.
Changing the lattice symmetry and allowing trees to be immune against fire, we
show that the critical exponents are universal.
