  Continuous phase transitions in spin systems can be formulated as percolation
of suitably defined clusters. We review this equivalence and then discuss how
in a similar way, the color deconfinement transition in SU(2) gauge theory can
be treated as a percolation phenomenon. In the presence of an external field,
spin systems cease to show thermal critical behavior, but the geometric
percolation transition persists (Kert\'esz line). For $H\not=0$, we study the
relation between percolation and pseudocritical behavior, both for continuous
and first order transitions, and show that it leads to the necessity of an
$H$-dependent cluster definition. A viable formulation of this kind could serve
as definition of deconfinement in QCD with dynamical quarks.
