OPEN (Object-oriented Process, Environment and Notation) is an international de facto standard object-oriented development method developed and maintained by the OPEN Consortium. OPEN consists of the OPEN Modeling Language (OML) as well as process, metrics, etc. This book specifies OML, a small but vital component of the complete OPEN method. It uses diagrams, tables, Web references and text to present the syntax, semantics and rationale behind OML. It documents version 1.0 of OML so that object-oriented modelers can learn and use it, and upperCASE vendors can support it.