  Reconfigurable intelligent surfaces (RISs) are regarded as a promising
emerging hardware technology to improve the spectrum and energy efficiency of
wireless networks by artificially reconfiguring the propagation environment of
electromagnetic waves. Due to the unique advantages in enhancing wireless
channel capacity, RISs have recently become a hot research topic. In this
article, we focus on three fundamental physical-layer challenges for the
incorporation of RISs into wireless networks, namely, channel state information
acquisition, passive information transfer, and low-complexity robust system
design. We summarize the state-of-the-art solutions and explore potential
research directions. Furthermore, we discuss other promising research
directions of RISs, including edge intelligence and physical-layer security.
