PYTHIA is a computer program that describes the collisions at high energies between particles such as e+, e-, p and pbar.  It is based on theory and phenomenological models of a number of physics aspects, including hard and soft interactions, parton distributions, initial- and final-state parton showers, multiple interactions, fragmentation and decay. It is largely based on original research, but also borrows many formulae and other knowledge from the literature.

Development of JETSET, the first member of the "Lund Monte Carlo" family, began in the Lund theory group in 1978, and evolved into one single program PYTHIA in 1997.   Until 2004, the FORTRAN code represented the main development branch, leading to PYTHIA 6.4.   This is currently maintained for legacy purposes only.

In 2004, development began on a new framework based on C++, and, with the release of PYTHIA 8.1 in 2007, the new code became the official main version.

The PYTHIA collaboration is a small group of theoretical physics who develop and maintain the computer code, including the Fermilab Scientist Stephen Mrenna , the Lund professor (and former Fermilab fellow) Torbjorn Sjostrand , the Monash professor (and former Fermilab post-doc) Peter Skands , post-doctoral researchers and students.   

PYTHIA is used by all high-energy physics experiments and the theoretical community to interpret data and make predictions at the energy frontier.   The PYTHIA manual was the most highly cited paper of 2012 according to SPIRES .

For information on Pythia8, please see the Pythia8 homepage

For information on Pythia6, follow the links on the Pythia8 homepage or view the Pythia6 Hepforge site