The articles in this special section focus on heterogeneous ultra-dense networks. In recent years, the rapid growth of various wireless communication services has led to an explosion of wireless data traffi c. Therefore, a major challenge in the fi fth generation (5G) mobile networks is to effectively serve the exponentially growing data fl ows in wireless networks. Initial estimations indicate that, diff erent from the evolutionary path of previous cellular generations that were based on spectral efficiency improvements, the most substantial amount of future system performance gains will be obtained by means of network infrastructure densification. In order to meet the requirements of explosive data traffic in 5G mobile communications, ultra-dense networking (UDN) has become a promising technology to significantly improve the network spectral effi ciency and system performance. Heterogeneous ultra-dense networking (HUDN) refers to the idea of densifying the cellular networks with very high network densifi - cation, including both the mobile device densification and base station (BS) densifi cation, where the density of BSs may exceed that of mobile devices. Therefore, UDNs can make the access nodes as close as possible to the users, resulting in efficient reuse of network resources while achieving the highest possible transmission rates.