Herwig++ is a general-purpose Monte Carlo event generator for simulating high-energy lepton-lepton, lepton-hadron, and hadron-hadron collisions, with a focus on accurate QCD radiation simulation. It builds on the HERWIG program, offering a more flexible structure and advanced features. The program includes a detailed treatment of QCD radiation suppression, BSM physics simulation, and a cluster-based hadronization model. It uses an eikonal model for the underlying event and simulates hadron decays via matrix elements, including spin correlations and off-shell effects. The simulation process involves several stages: hard subprocess generation, parton shower evolution, heavy object decay, multiple scattering, hadronization, and hadron decays. Herwig++ supports a wide range of processes, including QCD and electroweak interactions, and includes matrix elements for various hard processes. It also provides a Les Houches interface for external matrix element generators and handles incoming photons in hadron-hadron collisions. The program is structured using a modular framework based on ThePEG, with classes for event generation, parton showers, hadronization, and decay processes. It includes a variety of matrix elements for processes such as Drell-Yan production, Higgs boson production, and top quark production. The code allows users to customize parameters and models, with a focus on spin correlations and parton shower dynamics. Herwig++ supports next-to-leading-order (NLO) matrix elements and uses the POWHEG scheme for generating events with positive weights. It includes a Les Houches interface for external matrix element generators and handles incoming photons in hadron-hadron collisions. The program also provides a range of kinematic cuts and allows users to specify parameters for different processes. The code structure includes classes for event generation, parton showers, hadronization, and decay processes, with detailed documentation on the physics models and parameters. Herwig++ is distributed under the GNU General Public License (GPL) version 2 and includes guidelines for responsible use, including citation of relevant physics publications. The program is available for use and further development, with extensive documentation and examples provided.Herwig++ is a general-purpose Monte Carlo event generator for simulating high-energy lepton-lepton, lepton-hadron, and hadron-hadron collisions, with a focus on accurate QCD radiation simulation. It builds on the HERWIG program, offering a more flexible structure and advanced features. The program includes a detailed treatment of QCD radiation suppression, BSM physics simulation, and a cluster-based hadronization model. It uses an eikonal model for the underlying event and simulates hadron decays via matrix elements, including spin correlations and off-shell effects. The simulation process involves several stages: hard subprocess generation, parton shower evolution, heavy object decay, multiple scattering, hadronization, and hadron decays. Herwig++ supports a wide range of processes, including QCD and electroweak interactions, and includes matrix elements for various hard processes. It also provides a Les Houches interface for external matrix element generators and handles incoming photons in hadron-hadron collisions. The program is structured using a modular framework based on ThePEG, with classes for event generation, parton showers, hadronization, and decay processes. It includes a variety of matrix elements for processes such as Drell-Yan production, Higgs boson production, and top quark production. The code allows users to customize parameters and models, with a focus on spin correlations and parton shower dynamics. Herwig++ supports next-to-leading-order (NLO) matrix elements and uses the POWHEG scheme for generating events with positive weights. It includes a Les Houches interface for external matrix element generators and handles incoming photons in hadron-hadron collisions. The program also provides a range of kinematic cuts and allows users to specify parameters for different processes. The code structure includes classes for event generation, parton showers, hadronization, and decay processes, with detailed documentation on the physics models and parameters. Herwig++ is distributed under the GNU General Public License (GPL) version 2 and includes guidelines for responsible use, including citation of relevant physics publications. The program is available for use and further development, with extensive documentation and examples provided.