It is proved that the geometry of lightlike hypersurfaces of the de Si
tter space S-1(n+1) is directly connected with the geometry of hypersu
rfaces of the conformal space C-n. This connection is applied for a co
nstruction of an invariant normalization and an invariant affine conne
ction of lightlike hypersurfaces as well as for studying singularities
of lightlike hypersurfaces.