Ph.D / This thesis looks at current security problems within open system environments, that is security problems within heterogeneous computer system environments that are interconnected via computer networks. Thereafter two security models, Kerberos and the Path Context Model, are considered together with their respective ability to address these security problems. Using concepts of the Path Context Model, a new security model, called MOSS (Model for Open System Security), is developed and it is shown how MOSS can address all the security problems identified. Two possible implementations of MOSS are then considered: the one is based on the concept of Static Security Agents (SSAs) for contemporary open system environments, and the other is based on the concept of Roaming Security Agents (RSAs) for object orientated open system environments. The research is concluded with a summary of possible future research considerations