The initial deployment of 5G millimeter-Wave (mmWave) networks have shown that while they can sustain Gb/s wireless links with spatial multiplexing at low latencies, they are also highly susceptible to blockages, channel disruptions, and fading due to the nature of their directive beams. Robust mmWave coverage requires high densification of base stations that is prohibitively expensive and complex. Reconfigurable intelligent surfaces (RISs) have emerged as a technology to allow smart reconfiguration of the radio propagation environment by creating more favorable transmission characteristics. Constituted as an array of reconfigurable scattering or antenna elements, such passive surface (with near zero DC power consumption) are scalable and widely deployable. Traditionally referred to as reflect/transmit arrays in the microwave community, these arrays, when combined with silicon ICs in their new avatar, can now allow frequency scaling into the mmWave/THz, rapid programmability, scalability, amplification on-demand (for active surfaces), and sensing. Here, we present the case for such surfaces, design challenges, recent state-of-The-Art work, and their impact for future wireless networks.