This tutorial aims to help user interface designers and developers to understand the issues involved in multi-device interactive applications, which can be accessed through both mobile and stationary devices even exploiting different interaction modalities (graphical, vocal, gesture, ...). It will provide a discussion of the possible solutions in terms of concepts, techniques, languages, and tools, with particular attention to Web environments. The tutorial will deal with the various strategies in order to adapt the user interface according to the interaction resources available, also discussing what results can be obtained through model-based approaches when multi-device interfaces are considered. It will consider how to address such issues both when authoring multi-device interfaces and when user interfaces for different devices are dynamically adapted and can even migrate seamlessly across them to follow the mobile user. Thus, it will discuss how to support task continuity across multiple devices in examples of distributed and migratory interfaces and related usability issues.