  We describe a simple probabilistic method to cross-identify astrophysical
sources from different catalogs and provide the probability that a source is
associated with a source from another catalog or that it has no counterpart.
When the positional uncertainty in one of the catalog is unknown, this method
may be used to derive its typical value and even to study its dependence on the
size of objects. It may also be applied when the true centers of a source and
of its counterpart at another wavelength do not coincide.
  We extend this method to the case when there are only one-to-one associations
between the catalogs.
