The articles in this special issue focus on ultra-dense heterogeneous small cell deployment in 5G communications. Among enabling technologies for 5G communication systems and beyond, we are witnessing the explosive growth of ultra-dense heterogeneous deployment of small cells including micro eNBs, pico eNBs, femto eNBs, and remote radio heads (RRHs), driven by the stringent requirements of the fifth generation (5G) such as the increasing data rate, spectrum efficiency, and energy efficiency. The nature of an ultra-dense network, however, poses new challenges, not only in the deployment of massive base stations, but also in the frequent and complicated handovers, complex mobility management, QoE-ensured resource allocation, and inter-cell interference mitigation. These issues have to be carefully addressed to facilitate the successful deployment of ultra-dense heterogeneous networks. Toward this end, this Special Issue is devoted to presenting prominent research outcomes and the state-of-the-art advances in ultra-dense heterogeneous small cell deployment in emerging 5G and beyond.