The need to assess the effects of radiation on spacecraft is becoming increasingly important due to the increased sensitivity of microelectronics and sensors and the reduced availability of radiation hard alternatives. Monte Carlo (MC) simulation offers the ability to model in detail the physics of space radiation. Geant4 is a new-generation toolkit for Monte Carlo simulation, intended primarily for the high-energy physics community, but with potentially wider applications including space. Unlike other codes that attempt to treat the wide range of processes by interfacing existing MC models, Geant4 has been developed to provide, in a single tool, comprehensive treatment of the physical processes for a wide range of particles and energies. The European Space Agency as part of the Geant4 international collaboration is supporting the development of this code with the objective of making it the basis of a general space radiation shielding and effects tool.