Evolution and reactivity in the Semantic Web address the vision and concrete need for an active Web, where data sources evolve autonomously and perceive and react to events. In 2004, when the REWERSE project started, regarding work on Evolution and Reactivity in the Semantic Web there wasn't much more than a vision of such an active Web.

Materialising this vision requires the definition of a model, architecture, and also prototypical implementations capable of dealing with reactivity in the Semantic Web, including an ontology-based description of all concepts. This resulted in a general framework for reactive Event-Condition-Action rules in the Semantic Web over heterogeneous component languages.

Inasmuch as heterogeneity of languages is, in our view, an important aspect to take into consideration for dealing with the heterogeneity of sources and behaviour of the Semantic Web, concrete homogeneous languages targeting the specificity of reactive rules are of course also needed. This is especially the case for languages that can cope with the challenges posed by dealing with composite structures of events, or executing composite actions over Web data.

In this chapter we report on the advances made on this front, namely by describing the above-mentioned general heterogeneous framework, and by describing the concrete homogeneous language XChange.