  The Unified Modeling Language (UML) is rapidly emerging as a de-facto
standard for modelling OO systems. Given this role, it is imperative that the
UML needs a well-defined, fully explored semantics. Such semantics is required
in order to ensure that UML concepts are precisely stated and defined. In this
paper we motivate an approach to formalizing UML in which formal specification
techniques are used to gain insight into the semantics of UML notations and
diagrams and describe a roadmap for this approach. The authors initiated the
Precise UML (PUML) group in order to develop a precise semantic model for UML
diagrams. The semantic model is to be used as the basis for a set of
diagrammatical transformation rules, which enable formal deductions to be made
about UML diagrams. A small example shows how these rules can be used to verify
whether one class diagram is a valid deduction of another. Because these rules
are presented at the diagrammatical level, it will be argued that UML can be
successfully used as a formal modelling tool without the notational
complexities that are commonly found in textual specification techniques.
