  Integrated sensing and communications (ISAC) are envisioned to be an integral
part of future wireless networks, especially when operating at the
millimeter-wave (mmWave) and terahertz (THz) frequency bands. However,
establishing wireless connections at these high frequencies is quite
challenging, mainly due to the penetrating pathloss that prevents reliable
communication and sensing. Another emerging technology for next-generation
wireless systems is reconfigurable intelligent surfaces (RISs), which are
capable of modifying harsh propagation environments. RISs are the focus of
growing research and industrial attention, bringing forth the vision of smart
and programmable signal propagation environments. In this article, we provide a
tutorial-style overview of the applications and benefits of RISs for sensing
functionalities in general, and for ISAC systems in particular. We highlight
the potential advantages when fusing these two emerging technologies, and
identify for the first time that: i) joint sensing and communications designs
are most beneficial when the channels referring to these operations are
coupled, and that ii) RISs offer means for controlling this beneficial
coupling. The usefulness of RIS-aided ISAC goes beyond the individual obvious
gains of each of these technologies in both performance and power efficiency.
We also discuss the main signal processing challenges and future research
directions which arise from the fusion of these two emerging technologies.
