Herwig++ is a general-purpose Monte Carlo event generator designed for simulating high-energy lepton-lepton, lepton-hadron, and hadron-hadron collisions. It emphasizes accurate simulation of QCD radiation and includes features such as initial- and final-state QCD jet evolution, parton showering, hadronization, and underlying event simulation. The program is built on the ThePEG framework, allowing for modular implementation of physics models. Key features include: 1. **Hard Scattering Processes**: Simulation of various hard scattering processes, including leading-order and next-to-leading-order matrix elements. 2. **Les Houches Interface**: Integration with specialized matrix element generators for additional processes. 3. **Beyond the Standard Model (BSM) Physics**: Support for a range of BSM models, allowing for the addition of new models by encoding Feynman rules. 4. **Parton Showering**: Simulation of initial- and final-state QCD radiation, including color coherence effects, with a focus on heavy particle radiation. 5. **Underlying Event**: Simulated using an eikonal multiple parton-parton scattering model. 6. **Hadronization**: Description of hadron formation from quarks and gluons produced in the parton shower using the cluster hadronization model. 7. **Hadron Decays**: Simulation of hadron decays using matrix elements, including spin correlations and off-shell effects. The manual provides detailed technical descriptions, including the structure of the program, physics models, and code implementation. It also covers specific processes, such as matrix elements, perturbative decays, spin correlations, and QED radiation. Additionally, it discusses the framework for simulating BSM physics and the structure of the code for particle decays and QED radiation.Herwig++ is a general-purpose Monte Carlo event generator designed for simulating high-energy lepton-lepton, lepton-hadron, and hadron-hadron collisions. It emphasizes accurate simulation of QCD radiation and includes features such as initial- and final-state QCD jet evolution, parton showering, hadronization, and underlying event simulation. The program is built on the ThePEG framework, allowing for modular implementation of physics models. Key features include: 1. **Hard Scattering Processes**: Simulation of various hard scattering processes, including leading-order and next-to-leading-order matrix elements. 2. **Les Houches Interface**: Integration with specialized matrix element generators for additional processes. 3. **Beyond the Standard Model (BSM) Physics**: Support for a range of BSM models, allowing for the addition of new models by encoding Feynman rules. 4. **Parton Showering**: Simulation of initial- and final-state QCD radiation, including color coherence effects, with a focus on heavy particle radiation. 5. **Underlying Event**: Simulated using an eikonal multiple parton-parton scattering model. 6. **Hadronization**: Description of hadron formation from quarks and gluons produced in the parton shower using the cluster hadronization model. 7. **Hadron Decays**: Simulation of hadron decays using matrix elements, including spin correlations and off-shell effects. The manual provides detailed technical descriptions, including the structure of the program, physics models, and code implementation. It also covers specific processes, such as matrix elements, perturbative decays, spin correlations, and QED radiation. Additionally, it discusses the framework for simulating BSM physics and the structure of the code for particle decays and QED radiation.