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 crit-
ical exponent \u3bd 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 de-
localization proceeds through different renormalization patterns sharing the feature that
the boundary field becomes irrelevant in the delocalized regime. The boundary delocal-
ization (wetting) transition is shown to be continuous, with surface specific heat and layer
thickness exponents which can take values that we determine