This detailed and up-to-date description of heterogeneous cellular networking presents its characteristic features, the technology underpinning it, and the issues surrounding its use. Within this type of cellular system, special attention is paid to interference mitigation in self-organizing macrocell scenarios and femtocell deployments. Moreover, this work investigates the interference issues that arise when these two cell types are jointly deployed, complementing each other in what is currently known as a two-tier network. This book also provides new practical approaches to the inter-cell interference problem in both macrocell and femtocell OFDMA systems as well as in two-tier networks by means of the design of a novel framework and the use of mathematical optimization. Special attention is paid to the formulation of optimization problems and the development of well-performing solving methods (accurate and fast).