  We discuss the critical behavior of several three-dimensional magnetic
systems, such as pure and randomly dilute (anti)ferromagnets and stacked
triangular antiferromagnets. We also discuss the nature of the multicritical
points that arise in the presence of two distinct O(n)-symmetric order
parameters and, in particular, the nature of the multicritical point in the
phase diagram of high-T_c superconductors that has been predicted by the SO(5)
theory. For each system, we consider the corresponding Landau-Ginzburg-Wilson
field theory and review the field-theoretical results obtained from the
analysis of high-order perturbative series in the frameworks of the epsilon and
of the fixed-dimension d=3 expansions.
