A complete description of the PYTHIA program would have to cover four aspects:
The PYTHIA program is continuously being developed. We are aware of many shortcomings, some of which hopefully will be addressed in the future. No timetable is set up for such future changes, however. After all, this is not a professionally maintained software product, but part of a small physics research project. Very often, developments of the programs have come about as a direct response to the evolution of the physics stage, i.e. experimental results and studies for future accelerators. Hopefully, the program will keep on evolving in step with the new challenges opening up.
In the future, a radically new version of the program is required. Given the decisions by the big laboratories and collaborations to discontinue the use of Fortran and instead adopt C++, it is natural to attempt to move also event generators in that direction. The PYTHIA 7 project got going in the beginning of 1998, and was an effort to reformulate the event generation process in object oriented language [Lön99,Ber01]. However, after the HERWIG++ [Gie04] group decided to join in the development of a common administrative structure, much of the then PYTHIA 7 went into this new THEPEG [Lön05] framework, and work on the physics aspects stalled.
PYTHIA 8 is a clean new start, to provide a C++ successor to PYTHIA 6. In a return to the traditional PYTHIA spirit, it is a completely standalone generator, although some hooks for links to other programs will be provided. Work on PYTHIA 8 was begun from scratch in September 2004, with a three-year `road map' to produce a successor, but initially heavily biased towards physics and certainly far from complete in every respect. No attempt will be made to carry over all options of the existing code; rather the intent is to keep only the most recent and relevant physics models. The PYTHIA 6 and PYTHIA 8 versions therefore will coexist for several years, with the former rapidly approaching a steady state, and the latter continuing to be developed further. Thus, it is quite likely that the current manual will be the last major edition pertaining to the Fortran code, with future PYTHIA 6 developments only documented in update notes.