A major challenge for the next decade is to design virtual and augmented reality systems (VR at large) for real-world use cases such as healthcare, entertainment, e-education, and high-risk missions. This requires VR systems to operate at scale, in a personalized manner, remaining bandwidth-tolerant whilst meeting quality and latency criteria. One key challenge to reach this goal is to fully understand and anticipate user behaviours in these mixed reality settings. This can be accomplished only by a fundamental revolution of the network and VR systems that have to put the interactive user at the heart of the system rather than at the end of the chain. With this goal in mind, in this talk, we describe our current researches on user-centric systems. First, we describe our view-port based streaming strategies for 360-degree video. Then, we present more in details our research on of users‘ behaviour analysis, when users interact with the 360-degree content. Specifically, we describe a set of metrics that allows us to identify key behaviours among users and quantify the level of similarity of these behaviours. Specifically, we present our clique-based clustering methodology, information theory and trajectory base in-depth analysis. Finally, we conclude with an overview of the extension of this work to navigation within volumetric video sequences.