Radiation shielding is of primary importance in the planning of new interplanetary manned missions, and the ISS represents the largest laboratory for radiation shielding in space ever built. The ISS radiation environment has been thoroughly characterised by the Monte Carlo-based high-energy physics toolkit Geant4, finding that the ISS structure is quite effective at shielding from most forms of radiation, largely due to its internal arrangement. The internal environment found is consistent with literature values and astronaut dosimetry, and results indicate that the internal configuration of a spacecraft may be key to the radiation environment within it. Simulation results are then presented characterizing the interplanetary case, including detailed comparisons with other codes, with good agreement. Recommendations, guidelines and require- ments for interplanetary manned mission design are presented using these findings. These include prioritizing an SPE shelter, managing configuration and scheduling missions to avoid solar minimum to diminish GCRs. Alternative mitigation strategies such as active shielding are looked at, a magnetic shielding study is found to offer inferior protection to cheaper and lighter passive shielding. Maximum durations in interplanetary space for solar maximum and minimum under different shielding is es- timated and compared to NASA results. It is found that uncertainty in the biological effects of radiation conditions time in space and is a priority for future research.