With the wider use of ontologies in the Semantic Web and as part of production systems, multiple scenarios for ontology maintenance and evolution are emerging. For example, successive ontology versions can be posted on the (Semantic) Web, with users discovering the new versions serendipitously; ontology-development in a collaborative environment can be synchronous or asynchronous; managers of projects may exercise quality control, examining the changes from previous baseline versions and accepting or rejecting them before a new baseline is published, and so on. In this paper, we present the different scenarios for ontology maintenance and evolution that we have encountered in our own projects and in those of our collaborators. We define several dimensions that categorize different scenarios. For each scenario, we discuss the high-level tasks that an editing environment must support. We then present a unified comprehensive set of tools to support the different scenarios in a single framework, allowing users to switch between different modes easily.