The PYTHIA program is a powerful tool for generating high-energy physics events, particularly focusing on multiparticle production in collisions between elementary particles. The program aims to provide an accurate representation of event properties within and beyond the Standard Model, emphasizing strong interactions and multihadronic final states. The physics input covers various aspects such as hard subprocesses, parton showers, beam remnants, fragmentation, and decays. The manual provides extensive information on program elements, including subroutines, functions, switches, parameters, and particle and process data, allowing users to tailor the generation task to their specific interests. The program has evolved from the JETSET code, which was developed to study the hadronization process and string fragmentation model. PYTHIA has been extended to include a wide range of hard processes, initial- and final-state radiation, and physics beyond the Standard Model, such as supersymmetry, Technicolor, and extra dimensions. The program is versatile and can be used for various applications, including detector planning, analysis strategies, and interpreting experimental data. The manual also discusses the historical development of PYTHIA, the physics behind its components, and provides detailed instructions for using the program. It emphasizes the importance of understanding the limitations and approximations involved in the simulation of high-energy processes and encourages users to critically evaluate the results obtained from the program.The PYTHIA program is a powerful tool for generating high-energy physics events, particularly focusing on multiparticle production in collisions between elementary particles. The program aims to provide an accurate representation of event properties within and beyond the Standard Model, emphasizing strong interactions and multihadronic final states. The physics input covers various aspects such as hard subprocesses, parton showers, beam remnants, fragmentation, and decays. The manual provides extensive information on program elements, including subroutines, functions, switches, parameters, and particle and process data, allowing users to tailor the generation task to their specific interests. The program has evolved from the JETSET code, which was developed to study the hadronization process and string fragmentation model. PYTHIA has been extended to include a wide range of hard processes, initial- and final-state radiation, and physics beyond the Standard Model, such as supersymmetry, Technicolor, and extra dimensions. The program is versatile and can be used for various applications, including detector planning, analysis strategies, and interpreting experimental data. The manual also discusses the historical development of PYTHIA, the physics behind its components, and provides detailed instructions for using the program. It emphasizes the importance of understanding the limitations and approximations involved in the simulation of high-energy processes and encourages users to critically evaluate the results obtained from the program.