  There are three types of hypersurfaces in a pseudoconformal space C^n_1 of
Lorentzian signature: spacelike, timelike, and lightlike. These three types of
hypersurfaces are considered in parallel. Spacelike hypersurfaces are endowed
with a proper conformal structure, and timelike hypersurfaces are endowed with
a conformal structure of Lorentzian type. Geometry of these two types of
hypersurfaces can be studied in a manner that is similar to that for
hypersurfaces of a proper conformal space. Lightlike hypersurfaces are endowed
with a degenerate conformal structure. This is the reason that their
investigation has special features. It is proved that under the Darboux mapping
such hypersurfaces are transferred into tangentially degenerate
(n-1)-dimensional submanifolds of rank n-2 located on the Darboux hyperquadric.
The isotropic congruences of the space C^n_1 that are closely connected with
lightlike hypersurfaces and their Darboux mapping are also considered.
