The forthcoming fifthgeneration (5G) mobile wireless system is likely to lead to an increasingly heterogeneous data-demand pattern, including a small number of high-data-rate mobile broadband links and a large number of low-data-rate Internet of Things (IoT) applications. Aspects that govern seamless mobility between different access technologies, cell tiers, cell sectors, and frequency bands will be sensitive to the mobility model of the people, their devices, and the machines. An understanding of such entities will play a fundamental role in the future standardization and exploitation of the 5G technologies. In this article, we provide a comprehensive and general overview about existing mobility models that are useful to characterizing movement patterns across multiple distance and population scales. In doing so, all the models are critically discussed, providing strengths and weaknesses. Finally, open issues and critical design choices are highlighted to serve as guidelines for future research on this topic.