  With the surge of theoretical work investigating Reconfigurable Intelligent
Surfaces (RISs) for wireless communication and sensing, there exists an urgent
need of hardware solutions for the evaluation of these theoretical results and
further advancing the field. The most common solutions proposed in the
literature are based on varactors, Positive Intrinsic-Negative (PIN) diodes,
and Micro-Electro-Mechanical Systems (MEMS). This paper presents the use of
Liquid Crystal (LC) technology for the realization of continuously tunable
extremely large millimeter-wave RISs. We review the basic physical principles
of LC theory, introduce two different realizations of LC-RISs, namely
reflect-array and phased-array, and highlight their key properties that have an
impact on the system design and RIS reconfiguration strategy. Moreover, the LC
technology is compared with the competing technologies in terms of feasibility,
cost, power consumption, reconfiguration speed, and bandwidth. Furthermore,
several important open problems for both theoretical and experimental research
on LC-RISs are presented.
