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Summary form only given. The 1980s/1990s have witnessed an intense investigation of electromagnetic wave propagation phenomena at optical frequencies in periodic structures. These structures are usually referred to as photonic band gap (PBG) crystals. Despite the fact that most applications currently available make use of the linear properties of PBGs, the study of nonlinear dynamics has also been undertaken mainly in regard to soliton-like pulses (referred to as gap-solitons) in cubic /spl chi//sup (3)/, and quadratic /spl chi//sup (2)/ media within the context of one-dimensional multilayer stacks. Our aim is to describe by coupled mode equations the dynamics of the field in the PBG structure pointing out that the simultaneous availability of phase matching conditions and high density of modes (DOM) near the band edge lead to a strong enhancement of the quadratic response several order of magnitude greater respect to an equivalent length of perfect phase matched bulk materials.