Workflow is automation of a business process. A Workflow Management System (WFMS) is a system that assists in defining, managing and executing workflows. Most of the current WFMSs are built on traditional relational database systems and/or are using an object-oriented database system for storing the definition and run-time data about the workflows. However, to support flexible definition and adaptive features, such as on-line workflow evolution and exception handling, a WFMS requires advanced modeling functionality. In this paper, we describe our approach towards flexible workflow definition and workflow evolution, with a meta-modeling approach supported by an integrated advanced object environment (ADOME). Web-based mechanisms are also devised as part of the ADOME-WFMS system supporting workflow evolution.