The Semantic Web vision aims to bring machine-understandable (rather than machine-readable) content to the web user; this will allow the development of more sophisticated services and a better browsing experience by the end user. Ontologies are viewed as the key means through which the Semantic Web vision can be realized, as they can help in the representation of the content of a web page in a formal manner, so as to be suitable for use by an automated computer agent, crawler, search engine or other web service. Ontologies are dynamic entities which change over time. Often, it makes sense to record the changes that occurred between the different versions of ontologies for visualization or communication purposes, but such changes are often difficult or impossible to record as they happen. The focus of this talk is on the change detection problem which deals with the 'a posteriori' detection of changes. We describe a change language (and the corresponding detection algorithm) that allows the formulation of concise and intuitive deltas, which are expressive enough to describe unambiguously any possible change and that can be effectively and efficiently detected.