A Reconfigurable Intelligent Surface (RIS) is a planar structure that is engineered to have properties that enable the dynamic control of the electromagnetic waves. In wireless communications and networks, RISs are an emerging technology for realizing programmable and reconfigurable wireless propagation environments through nearly passive and tunable signal transformations. RIS-assisted programmable wireless environments are a multidisciplinary research endeavor but are not well-understood. This special issue is aimed to lead to a widespread dissemination of state-of-the-art analytical and algorithmic tools, testbed implementations and experimental activities, research perspectives, and to enable the acceleration in the germination of novel ideas pertaining to the development of RISs for various applications in wireless communications and networks. This special issue will include contributions from world-renowned experts in physics, electromagnetics, antennas and propagation, microwave theory and techniques, metamaterials, circuit design, devices, wireless communications, wireless networks, computer science, signal processing, localization and sensing, optimization theory, machine learning and data science, to provide a unified platform where modern state-of-the-art research ideas and long-term research directions are brought together.