The project Management Institute`s Guide to the Project Management Body of Knowledge (PMBOK) underpins many initiatives to improve project management practice. It is widely used for training and underpins the development of competency standards. Because of its fundamental importance, the PMBOK should be critically reviewed. This paper agues for an expansion of the MPBOK Guide`s risk management knowledge area to include a wider perspective of incomplete knowledge. The PMBOK Guide deals with uncertainty through the traditional use of probability theory, however the underpinning assumptions of probability theory do not always apply in practice. Furthermore, probably-based risk management theory does not explain important aspects of observed project management pratice. This paper discusses an expanded framework of incomplete knowledge, including: an expanded concept of uncertainty that acknowledges ignorance or surprise, where there is no prior knowledge of future states; imprecision arising from ambiguity (fuzziness) in project parameters and future states; and, human limitations in information processing. The paper shows the expanded framework explains oberved project-management practice more thorgoughly than teh probability-based framework. The conclusion reached is that "management of incomplete knowledge" provides a better theoretical foundation for improving project management practice. To substantiate this, a promising new approach based on "real option" is also discussed