  Current discussions on the sixth Generation (6G) of wireless communications
are envisioning future networks as a unified communication, sensing, and
computing platform that intelligently enables diverse services, ranging from
immersive to mission critical applications. The recently conceived concept of
the smart radio environment, enabled by Reconfigurable Intelligent Surfaces
(RISs), contributes towards this intelligent networking trend, offering
programmable propagation of information-bearing signals, which can be jointly
optimized with transceiver operations. Typical RIS implementations include
metasurfaces with nearly passive meta-atoms, allowing to solely reflect the
incident wave in an externally controllable way. However, this purely
reflective nature induces significant challenges in the RIS orchestration from
the wireless network. For example, channel estimation, which is essential for
coherent communications in RIS-empowered wireless networks, is quite
challenging with the available RIS designs. This article introduces the concept
of Hybrid reflecting and sensing RISs (HRISs), which enables metasurfaces to
reflect the impinging signal in a controllable manner, while simultaneously
sense a portion of it. The sensing capability of HRISs facilitates various
network management functionalities, including channel estimation and
localization. We discuss a hardware design for HRISs and detail a full-wave
proof-of-concept. We highlight their distinctive properties in comparison to
reflective RISs and active relays, and present a simulation study evaluating
the HRIS capability for performing channel estimation. Future research
challenges and opportunities arising from the concept of HRISs are presented.
