It provides a complete set of tools for all areas of detector simulation: geometry, tracking, detector response, run, event and track management, visualization and user interface. The multi-disciplinary nature of the toolkit requires that it supply an abundant set of Physics Processes to handle diverse interactions of particles with matter over a wide energy range. For many physics processes a choice of different models is available.

A large set of utilities is provided, including a set of physics units and constants, particle management compliant with the Particle Data Group, interfaces to event generators, and object persistency solutions, complete the toolkit.

Geant4 exploits advanced software engineering techniques and object-oriented technology to achieve transparency of the physics implementation and hence provide the possibility of validating the physics results.

User requirements were collected in the initial phase of the project and are periodically updated.

Problem domain decomposition and object-oriented analysis and design have led to a clear hierarchical structure of sub-domains, linked by a uni-directional flow of dependencies.

The object-oriented design of Geant4 allows the user to understand, customize or extend the toolkit in all the domains. At the same time, its modular architecture allows the user to load and use only the components needed.

Geant4 software was developed in the context of the CERN RD44 R&D project, as a world-wide collaboration of about 100 scientists participating in more than 10 experiments in Europe, Russia, Japan, Canada and the United States. The achievements of the development phase are summarized in the RD44 final Status Report (S.Giani et al., CERN/LHCC-98-44, 1998) to the LCB.

Since 1999 the production service, user support and development of Geant4 have been managed by the international Geant4 Collaboration. The collaboration is based on a Collaboration Agreement among the participating laboratories, experiments and national institutes. Many specialized working groups are responsible for the various domains of the toolkit.

The Geant4 source code is freely available, accompanied by an Installation Guide and an extensive set of documentation.