  Reconfigurable intelligent surfaces (RISs) are considered as potential
technologies for the upcoming sixth-generation (6G) wireless communication
system. Various benefits brought by deploying one or multiple RISs include
increased spectrum and energy efficiency, enhanced connectivity, extended
communication coverage, reduced complexity at transceivers, and even improved
localization accuracy. However, to unleash their full potential, fundamentals
related to RISs, ranging from physical-layer (PHY) modelling to RIS phase
control, need to be addressed thoroughly. In this paper, we provide an overview
of some timely research problems related to the RIS technology, i.e., PHY
modelling (including also physics), channel estimation, potential RIS
architectures, and RIS phase control (via both model-based and data-driven
approaches), along with recent numerical results. We envision that more efforts
will be devoted towards intelligent wireless environments, enabled by RISs.
