  We numerically investigate the necessary ingredients for reentrant behavior
in the phase diagram of physical systems. Studies on the possibly simplest
model that exhibits reentrance, the two-dimensional random bond Ising model,
show that reentrant behavior is generic whenever frustration is present in the
model. For both discrete and continuous disorder distributions, the phase
diagram in the disorder-temperature plane is found to be reentrant, where for
some disorder strengths a paramagnetic phase exists at both high and low
temperatures, but an ordered ferromagnetic phase exists for intermediate
temperatures.
