  The emergence of various technologies demanding both high data rates and
precise sensing performance, such as autonomous vehicles and internet of things
devices, has propelled an increasing popularity of integrated sensing and
communication (ISAC) in recent years. ISAC offers an efficient framework for
communication and sensing where both functionalities are carried out in a
shared spectrum, utilizing the same hardware, beamformer and waveform design.
At the same time, intelligent metasurfaces have been identified as an
architectural enabler for the upcoming sixth-generation (6G) of wireless
communication due to their ability to control the propagation environment in an
energy-efficient manner. Due to the potential of metasurfaces to enhance both
communication and sensing performance, numerous papers have explored the
performance gains of using metasurfaces to improve ISAC. This survey reviews
the existing literature on metasurface-assisted ISAC, detailing the associated
challenges and opportunities. To provide a comprehensive overview, we commence
by offering relevant background information on standalone metasurface-assisted
communication and metasurface-assisted sensing systems, followed by a
discussion on the fundamentals of ISAC. The core part of the paper then
summarizes the state-of-the-art studies on metasurface-assisted ISAC with
metasurfaces employed as separate entities placed between the transmitter and
receiver, also known as reconfigurable intelligent surfaces, with an emphasis
on its two levels of integration: radio-communications co-existence and
dual-function radar-communications. We also review the current works in the
area of holographic ISAC where metasurfaces are used to form part of ISAC
transmitter. Within each category, the challenges, opportunities and future
research directions are also highlighted.
