In designing and developing large complex products, people use models to describe and organize interrelated elements in both product systems (architecture, use cases, constraints...) and process systems (activities, deliverables, roles...). However, exchanged information is often incomplete, vague and not entirely determined at the beginning of the project and during its evolution. Our project is to improve predictability and stability of product development (PD) processes by proposing a framework to identify and manage uncertainty and imprecision. As a first step, we focus on modeling uncertain and imprecise information in PD process modeling. Drawing on our industrial experiences and literature reviews where we found that 112 elements1 often brought uncertainty and imprecision, we distinguish four types of uncertainty and imprecision. We then propose a simple, structured modeling framework to describe these elements - a new Unified Modeling Language (UML) profile .