  We report our Monte Carlo results on the critical and multicritical behavior
of the +- J Ising model [with a random-exchange probability P(J_{xy}) = p
\delta(J_{xy} - J) + (1-p) \delta(J_{xy} + J)], in two and three dimensions. We
study the transition line between the paramagnetic and ferromagnetic phase,
which extends from p=1 to a multicritical (Nishimori) point. By a finite-size
scaling analysis, we provide strong numerical evidence that in three dimensions
the critical behavior along this line belongs to the same universality class as
that of the critical transition in the randomly dilute Ising model. In two
dimensions we confirm that the critical behavior is controlled by the pure
Ising fixed point and that disorder is marginally irrelevant, giving rise to
universal logarithmic corrections. In both two and three dimensions, we also
determine the location of the multicritical Nishimori point, as well as the
renormalization-group dimensions of the operators that control the
renormalization-group flow close to it.
