  In the framework of the renormalization-group (RG) approach, critical
phenomena can be investigated by studying the RG flow of multi-parameter
$\Phi^4$ field theories with an $N$-component fundamental field, containing up
to 4th-order polynomials of the field. Some physically interesting systems
require $\Phi^4$ field theories with several quadratic and quartic parameters,
depending essentially on their symmetry and symmetry-breaking pattern at the
transition. Results for their RG flow apply to disorder and/or frustrated
systems, anisotropic magnetic systems, density wave models, competing orderings
giving rise to multicritical behaviors. The general properties of the RG flow
in multi-parameter $\Phi^4$ field theories are discussed. An overview of
field-theoretical results for some physically interesting cases is presented,
and compared with other theoretical approaches and experiments. Finally, this
RG approach is applied to investigate the nature of the finite-temperature
transition of QCD with $N_f$ light quarks.
