  The PYTHIA program can be used to generate high-energy-physics 'events' with
sets of outgoing particles produced in the interactions between two incoming
particles. The objective is to provide a representation, as accurate as
possible, of event properties in a wide range of reactions. One such reaction,
that is not well understood is Diffraction. Among the several alternative
approaches that have been proposed, in PYTHIA, we follow a fairly conventional
Pomeron based one, but fully integrated to use the standard PYTHIA machinery
for multiple interactions, parton showers and hadronization. This note reports
the development in PYTHIA in the way diffraction is modeled without providing
specific details for usage. Results are compared with an alternative event
generator called PHOJET. The code and further information may be found on the
Pythia web page: http://home.thep.lu.se/~torbjorn/Pythia.html.
