Computer networks are now an integral part of many businesses and they are expected to satisfy user requirements. However, there are now many different technologies available, and creating a reliable, high performance system is far from trivial. Issues to be resolved include performance and reliability levels, technology, compatibility, expandability and legacy systems. It is expensive to contract an expert to consider all options available for every network installation. This thesis addresses the issue of identifying an effective network solution for a low cost. Designing a network relies on a sequence of hierarchical decisions, which is amenable to an expert systems approach. Software has been developed to present the client a series of questions to enable a good networking solution to be derived. However, the best network solution, measured in terms of performance and cost, cannot be derived from a simple hierarchical decision structure. The performance of the network needs to be assessed using computer modelling. Computer modelling techniques have been used to simulate the performance of proposed network solutions.