Reactivity on the Web is an emerging issue. It is essential for upcoming Web systems such as online marketplaces, adaptive, Semantic Web systems as well as Web services and Grids. This article first introduces the paradigms upon which the high-level language XChange for programming reactive behaviour and distributed applications on the Web relies. Then, it briefly presents the main syntactical constructs of XChange. Finally, it sketches the implementation in XChange of a reactive Web-based application. Categories and Subject Descriptors D.3.3 [Software]: Programming Languages--Language Constructs and Features Keywords Web, reactive languages, event-condition-action rules