Plastic scintillators exhibiting pulse shape discrimination properties represent a solid-state alternative to the use of organic liquids and crystals for the detection of neutron and gamma radiation. They are robust, inexpensive and can be fabricated in a variety of shapes and sizes. The time dependent pulse shapes derived from organic scintillation detectors can be characterised by a rising edge and multiple decay time constants. These time constants relate to the scintillation mechanism following particle interaction and differ depending on the type of interacting particle. The technique of pulse shape discrimination enables the separation of neutron and gamma ray induced signals based on subtle differences in their pulse shape. The accurate simulation of pulse shapes presents the opportunity to assess the pulse shape discrimination performance of scintillation detectors prior to fabrication. Although the pulse shapes from organic and inorganic scintillation detectors have been modelled, current methods have either excluded the influence of the photodetector or employed simple analytical approximations that do not provide a fully accurate representation of the response. The objective of this research is to develop accurate pulse shape simulations capable of reproducing the pulses measured experimentally for organic scintillation detectors. Work presented demonstrates the use of the Monte Carlo toolkit GEANT4 to simulate the time-dependent pulse shapes from a EJ-267 plastic scintillator coupled to a ET-Enterprises 9214 photomultiplier tube. GEANT4 has been used to simulate the generation and transportation of scintillation photons up to their detection at the photocathode. This represents the pulse shape when the influence of the photodetector is excluded. The response of the photomultiplier tube has been obtained experimentally using a pulsed light source and then integrated with the simulation. Future work will involve testing this method using different photomultiplier tubes and silicon photomultipliers, which are increasingly used as the photodetector of choice for novel scintillation detectors. The overall aim is the creation of a GEANT4 library of photodetector responses, which we are working on with colleagues in Japan. This could lead to a step change in the simulation of scintillation detectors, simplifying the process and enabling the ability to evaluate the pulse shape discrimination performance of advanced scintillation detectors entirely within the GEANT4 framework.