  The future of mobile communications looks exciting with the potential new use
cases and challenging requirements of future 6th generation (6G) and beyond
wireless networks. Since the beginning of the modern era of wireless
communications, the propagation medium has been perceived as a randomly
behaving entity between the transmitter and the receiver, which degrades the
quality of the received signal due to the uncontrollable interactions of the
transmitted radio waves with the surrounding objects. The recent advent of
reconfigurable intelligent surfaces in wireless communications enables, on the
other hand, network operators to control the scattering, reflection, and
refraction characteristics of the radio waves, by overcoming the negative
effects of natural wireless propagation. Recent results have revealed that
reconfigurable intelligent surfaces can effectively control the wavefront,
e.g., the phase, amplitude, frequency, and even polarization, of the impinging
signals without the need of complex decoding, encoding, and radio frequency
processing operations. Motivated by the potential of this emerging technology,
the present article is aimed to provide the readers with a detailed overview
and historical perspective on state-of-the-art solutions, and to elaborate on
the fundamental differences with other technologies, the most important open
research issues to tackle, and the reasons why the use of reconfigurable
intelligent surfaces necessitates to rethink the communication-theoretic models
currently employed in wireless networks. This article also explores theoretical
performance limits of reconfigurable intelligent surface-assisted communication
systems using mathematical techniques and elaborates on the potential use cases
of intelligent surfaces in 6G and beyond wireless networks.
