  We study spatial optical solitons in a one-dimensional nonlinear photonic
crystal created by an array of thin-film nonlinear waveguides, the so-called
Dirac-comb nonlinear lattice. We analyze modulational instability of the
extended Bloch-wave modes and also investigate the existence and stability of
bright, dark, and ``twisted'' spatially localized modes in such periodic
structures. Additionally, we discuss both similarities and differences of our
general results with the simplified models of nonlinear periodic media
described by the discrete nonlinear Schrodinger equation, derived in the
tight-binding approximation, and the coupled-mode theory, valid for shallow
periodic modulations of the optical refractive index.
