Wireless communications technology enables us to seamlessly access many multimedia services, e.g., stored multimedia (e.g., video on-demand), live streaming (e.g., Internet live sport networks, Internet radio stations), and real-time interactive streaming (e.g., online games, video conference, e-education), etc. As such, wireless communications technology has rapidly gained a crucial role and become an important aspect of life. Of most concern, however, is the increase in wireless/mobile devices and the huge demand in data rates associated with this. The future 5G cellular network is expected to achieve as much as 1000 times data rate relative to its current 4G counterpart. Data rates are projected to increase by a factor of ten every five years, and with the emerging Internet of Things (IoT) predicted to wirelessly connect trillions of devices across the globe, without novel approaches, future mobile networks (5G) will grind to a halt unless more capacity is created. One of the solutions is the implementation of ultra-dense networks, combining macro-cells and small-cells and exploiting the emerging technologies of millimetre wave (mm-wave) frequency bands and large-scale antennas arrays. The aim of this book is to present new research related to theory or practice of all aspects of ultra-dense networks.