We extend the concept of cross-device user interfaces into the new, more general, concept of omni-channel user interfaces to better reflect the technological variety offered for developing multi-target user interfaces for interactive applications. We present a model-based approach for developing runtime omni-channel user interfaces for multi-target applications, which consists of: (1) OpenUIDL, a user interface description language for describing omni-channel user interfaces with its semantics by a meta-model and its syntax based on JSON, (2) the definition of a step-wise approach for producing runtime interactive applications based onOpenUIDLwith integration into the development life cycle, (3) the development of a cloud-based, OpenUIDL compliant, Interactive Development Environment that supports the application and the enactment of the step-wise approach and its illustration on several multi-target user interfaces.