  The rapid advancement of wireless communication technologies has precipitated
an unprecedented demand for high data rates, extremely low latency, and
ubiquitous connectivity. In order to achieve these goals, stacked intelligent
metasurfaces (SIM) has been developed as a novel solution to perform advanced
signal processing tasks directly in the electromagnetic wave domain, thus
achieving ultra-fast computing speed and reducing hardware complexity. This
article provides an overview of the SIM technology by discussing its hardware
architectures, advantages, and potential applications for wireless sensing and
communication. Specifically, we explore the utilization of SIMs in enabling
wave-domain beamforming, channel modeling and estimation in SIM-assisted
communication systems. Furthermore, we elaborate on the potential of utilizing
a SIM to build a hybrid optical-electronic neural network (HOENN) and
demonstrate its efficacy by examining two case studies: disaster monitoring and
direction-of-arrival estimation. Finally, we identify key implementation
challenges, including practical hardware imperfections, efficient SIM
configuration for realizing wave-domain signal processing, and performance
analysis to motivate future research on this important and far-reaching topic.
