The goal of software testing is to detect defects with the objective of removing them at a later stage in the development process. Interactive software development follows the User Centered Design approach that promotes continuous involvement of users both at design and evaluation phases. This process is meant to produce usable interactive software by gathering functional and non-functional requirements related to both user needs and context of use. However, taking into account these potentially very-complex-to-implement requirements increases the complexity of the software that is likely, without appropriate methods and tools, to encompass a large number of defects. One of the limitations of UCD approaches is that it provides no guidance on the engineering of the interactive application, which thus usually embeds numerous defects resulting in failures at the origin of user frustrations and performance drops. Even though a classification of interactive application defects has been proposed, interactive application testers remain only superficially supported in detecting them. This paper defines a model-based approach to engineer the testing activity for interactive applications. It proposes a process that bridges the gap between UCD artefacts and interactive software implementation by the production of a dedicated formal model exploited for testing purposes only. The application of the process is demonstrated on an interactive cockpit WIMP application. Finally, threats to validity (capability of the approach to detect defects and to ensure an acceptable coverage testing of the interactive application) are addressed by a longitudinal study on 61 variants of a simple application developed by 61 different developers. ?