  This manual describes the PYTHIA 8.3 event generator, the most recent version
of an evolving physics tool used to answer fundamental questions in particle
physics. The program is most often used to generate high-energy-physics
collision "events", i.e. sets of particles produced in association with the
collision of two incoming high-energy particles, but has several uses beyond
that. The guiding philosophy is to produce and reproduce properties of
experimentally obtained collisions as accurately as possible. The program
includes a wide ranges of reactions within and beyond the Standard Model, and
extending to heavy ion physics. Emphasis is put on phenomena where strong
interactions play a major role. The manual contains both pedagogical and
practical components. All included physics models are described in enough
detail to allow the user to obtain a cursory overview of used assumptions and
approximations, enabling an informed evaluation of the program output. A number
of the most central algorithms are described in enough detail that the main
results of the program can be reproduced independently, allowing further
development of existing models or the addition of new ones. Finally, a chapter
dedicated fully to the user is included towards the end, providing pedagogical
examples of standard use cases, and a detailed description of a number of
external interfaces. The program code, the online manual, and the latest
version of this print manual can be found on the PYTHIA web page:
https://www.pythia.org/
