  The theory of interface localization in near-critical planar systems at phase
coexistence is formulated from first principles. We show that mutual
delocalization of two interfaces, amounting to interfacial wetting, occurs when
the bulk correlation length critical exponent $\nu$ is larger than or equal to
1. Interaction with a boundary or defect line involves an additional scale and
a dependence of the localization strength on the distance from criticality. The
implications are particularly rich in the boundary case, where delocalization
proceeds through different renormalization patterns sharing the feature that
the boundary field becomes irrelevant in the delocalized regime. The boundary
delocalization (wetting) transition is shown to be continuous, with surface
specific heat and layer thickness exponents which can take values that we
determine.
