Project-based professional service organisations supply their services as tailored or one-off projects for specific clients. The particular form of their organisation, the character of their relationships with their clients necessary to deliver highly customised projects and the non-routine, creative nature of the work come together in a way which makes the management of these service firms particularly demanding. A common challenge is fluctuation in the workload. While this is partly influenced by changes in demand, the external environment does not provide a comprehensive explanation and the interaction between business processes and project processes needs to be examined. In providing a generic explanation of the causes of workload fluctuation as well as an assessment of different bidding strategies based on a system dynamics model, this paper aims to help to advance the theoretical understanding of the project-based professional service organisation and ultimately to help to provide tools for its managers.