The Protg project has come a long way since Mark Musen first built the Protg metatool for knowledge-based systems in 1987. The original tool was a small application, aimed at building knowledge-acquisition tools for a few specialized programs in medical planning. From this initial tool, the Protg system has evolved into a durable, extensible platform for knowledge-based systems development and research. The current version, Protg-2000, can be run on a variety of platforms, supports customized user-interface extensions, incorporates the Open Knowledge Base Connectivity (OKBC) knowledge model, interacts with standard storage formats such as relational databases, XML, and RDF, and has been used by hundreds of individuals and research groups. In this paper, we follow the evolution of the Protg project through 3 distinct re-implementations. We describe our overall methodology, our design decisions, and the lessons we have learned over the duration of the project.. We believe that our success is one of infrastructure: Protg is a flexible, well-supported, and robust development environment. Using Protg, developers and domain experts can easily build effective knowledge-based systems, and researchers can explore ideas in a variety of knowledge-based domains.

sumes that, as with Web browsers, users will want to enhance and custom tailor the system's behavior by means of a variety of "plugins. " These plug-ins are modular pieces of program code that add new functionalities to Protg-2000 in well circumscribed ways. Developers can contribute new Protg-2000 plug-ins to a library maintained on the Internet, and can freely download new plug-ins to augment the behavior of their own knowledge-acquisition systems constructed using Protg-2000. Our approach establishes a new kind of knowledge-acquisition enterprise---one of building knowledge-acquisition--tool components that can be shared among a large community of users over the Internet. Protg-2000's modular architecture expands tremendously the kinds of systems that can be assembled to address specific knowledgeacquisition tasks, and offers the possibility that future knowledge-acquisition systems can be better tailored to the particular requirements of end users. 1. The Protg Lineage Since the

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