This PhD thesis focuses on plasticity for Mixed Reality (MR) User interfaces, which includesVirtual Reality (VR), Augmented Reality (AR) and Augmented Virtuality (AV) applications.Plasticity refers to the capacity of an interactive system to withstand variations of both the systemphysical characteristics and the environment while preserving its usability. Usability continuity ofa plastic interface is ensured whatever the context of use. Therefore, we propose a set of softwaremodels, integrated in a software solution named 3DPlasticToolkit, that allow any developer tocreate plastic MR user interfaces. First, we propose three models for modeling adaptation sources:a model for the description of display devices and interaction devices, a model...