  The latest discussions on the upcoming sixth Generation (6G) of wireless
communications are envisioning future networks as a unified communications,
sensing, and computing platform. The recently conceived concept of the smart
radio environment, enabled by Reconfigurable Intelligent Surfaces (RISs),
contributes towards this vision offering programmable propagation of
information-bearing signals. Typical RIS implementations include metasurfaces
with almost passive unit elements capable of reflecting their incident waves in
controllable ways. However, this solely reflective operation induces
significant challenges for the RIS optimization from the wireless network
orchestrator. For example, RISs lack information to locally tune their
reflection pattern, which can only be acquired by other network entities, and
then shared with the RIS controller. Furthermore, channel estimation, which is
essential for coherent RIS-empowered communications, is challenging with the
available RIS designs. This article reviews the emerging concept of Hybrid
reflecting and sensing RISs (HRISs), which enables metasurfaces to reflect the
impinging signal in a controllable manner, while simultaneously sensing a
portion of it. The sensing capability of HRISs facilitates various network
management functionalities, including channel parameter estimation and
localization, while giving rise to potentially computationally autonomous and
self-configuring metasurfaces. We discuss a hardware design for HRISs and
detail a full-wave electromagnetic proof of concept. The distinctive properties
of HRISs, in comparison to their solely reflective counterparts, are
highlighted and a simulation study evaluating their capability for performing
full and parametric channel estimation is presented. Future research challenges
and opportunities arising from the HRIS concept are also included.
