Cluster Percolation and Thermal Critical Behaviour

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.