As ontology development becomes a collaborative process, developers face the problem of maintaining versions of ontologies akin to maintaining versions of software code or versions of documents in large projects. Traditional versioning systems enable users to compare versions, examine changes, and accept or reject changes. However, while versioning systems usually treat software code and text documents as text files, a versioning system for ontologies must compare and present structural changes rather than changes in text representation of ontologies. In this paper, we present the PromptDiff ontology-versioning environment, which address these challenges. PromptDiff includes an efficient version-comparison algorithm that produces a structural diff between ontologies. The results are presented to the users through an intuitive user interface for analyzing the changes that enables users to view concepts and groups of concepts that were added, deleted, and moved, distinguished by their appearance and with direct access to additional information characterizing the change. The users can then act on the changes, accepting or rejecting them. We present results of a pilot user study that demonstrate the effectiveness of the tool for change management. We discuss design principles for an end-to-end ontology-versioning environment and position ontology versioning as a component in a general ontology-management framework.