These ideas first appeared in two studies done independently at CERN and KEK in 1993. Both groups sought to investigate how modern computing techniques could be applied to improve the existing FORTRAN based Geant3 simulation program. Activities were merged in the fall of 1994 and a formal proposal, RD44, to construct an entirely new program based on object-oriented technology was submitted to CERN's Detector Research and Development Committee. The initiative grew to become a large international collaboration of physicist programmers and software engineers from a number of institutes and universities participating in a range of high-energy physics experiments in Europe, Japan, Canada and the United States. The objective was to write a detector simulation program which had the functionality and flexibility necessary to meet the requirements of the next generation of subatomic physics experiments. The initial scope quickly widened when it became apparent that such a tool would also benefit the nuclear, accelerator, space and medical physics community, with more individuals joining from these fields of science.

The RD44 project represented a pioneering effort in redesigning a major CERN software package for a modern object-oriented (OO) environment based on C++. The R&D phase was completed in December 1998 with the delivery of the first production release. The collaboration was subsequently renamed Geant4 and reinstated on the basis of a formal Memorandum of Understanding (MoU) signed by many of the same national institutes, laboratories and large HEP experiments who participated in RD44. The agreement addresses the program management, maintenance and user support during the production phase and the continued development and refinement of the toolkit. It is subject to tacit renewal every two years and sets out a collaboration structure defined by a Collaboration Board (CB), a Technical Steering Board (TSB) and several working groups. In February 2006 a new MoU came into effect and CB and TSB were renamed to Oversight Board (OB) and Steering Board (SB) respectively.

The collaboration now profits from the accumulated experience of many contributors to the field of Monte Carlo simulation of physics detectors and physical processes. While geographically distributed software development and large-scale object-oriented systems are no longer a novelty, Geant4, in terms of the size and scope of the code and the number of contributors, may well represent the largest and most ambitious project of its kind outside the corporate world. A clean overall problem decomposition has led to a clear hierarchical structure of domains. Every section of the Geant4 software, which corresponds to a releasable component (library), is individually managed by a working group of experts. In addition, there is a working group for each of the activities: testing and quality assurance, software management and documentation management. A release coordinator heads each group. This consequent distribution of responsibility among a relative large number of people permits a support structure whereby outside users can address questions directly to the appropriate expert.