Assumptions affect our everyday life and more specifically in business they can have a profound effect on an organisation's support services delivery performance if they prove to be wrong. The literature shows the need to identify assumptions and expose any implicit assumptions that may have a major impact on the business objectives. There are also examples of methodologies that suggest there are advantages in managing assumptions for strategic decision making. However, there is little research dealing with the ongoing assessment or review of assumptions when delivering support services during the In-Service phase of a product's lifecycle. The outputs of various government audit reports and independent enquiries suggest that erroneous assumptions are a potential cause of death or serious injury and substantial increases in costs or reductions in capability of major capital systems. This suggests that a method for the ongoing review and management of assumptions would be beneficial as an aid to the successful delivery of support services. An industry project was observed to gain an insight into how projects deal with assumptions. A number of methods from the strategic planning, risk management and reliability engineering domains are compared and these form the basis for a proposed process to manage assumptions in the In-service phase of a system's lifecycle.