The book focuses on the modeling, analysis and optimization of Automatic Retransmission reQuest (ARQ) protocols as part of a wireless communications system. The work considers systems in increasing complexity and is divided into two parts. The first considers single link communications while the second investigates point-to-multipoint systems and wireless networks. The first part provides a thorough discussion on performance measures in ARQ communications. A definition for reliable protocols and conditions to achieve this are provided. In case that, due to delay constraints, reliability cannot be guaranteed, conditions for the optimal truncation of the ARQ chain are derived, considering both ARQ protocols in isolation as well as at the server of a single queue. The second part investigates power control policies to maximize the amount of data being transfered at the downlink of a cellular communications system as well as at a network with per hop retransmissions, taking interference between nodes into account. The stability regions of such systems is explicitly provided, while algorithms that can achieve this are provided and their performance is analyzed.