  The theory of phase transitions is based on the consideration of "idealized"
models, such as the Ising model: a system of magnetic moments living on a cubic
lattice and having only two accessible states. For simplicity the interaction
is supposed to be restricted to nearest--neighbour sites only. For these
models, statistical physics gives a detailed description of the behaviour of
various thermodynamic quantities in the vicinity of the transition temperature.
These findings are confirmed by the most precise experiments. On the other
hand, there exist other cases, where one must account for additional features,
such as anisotropy, defects, dilution or any effect that may affect the nature
and/or the range of the interaction. These features may have impact on the
order of the phase transition in the ideal model or smear it out. Here we
address two classes of models where the nature of the transition is altered by
the presence of anisotropy or dilution.
