is paper introduces the Smart Home Simulator, one of the main outcomes of the D4All project. is application takes into account the variety of issues involved in the development of Ambient Assisted Living (AAL) solutions, such as the peculiarity of each endusers, appliances, and technologies with their deployment and data-sharing issues. e Smart Home Simulator-a mixed reality application able to support the con guration and customization of domestic environments in AAL systems-leverages on integration capabilities of Semantic Web technologies and the possibility to model relevant knowledge (about both the dwellers and the domestic environment) into formal models. It also exploits Virtual Reality technologies as an e cient means to simplify the con guration of customized AAL environments. e application and the underlying framework will be validated through two di erent use cases, each one foreseeing the customized con guration of a domestic environment for speci c segments of users.