  The increasing demand for wireless data transfer has been the driving force
behind the widespread adoption of Massive MIMO (multiple-input multiple-output)
technology in 5G. The next-generation MIMO technology is now being developed to
cater to the new data traffic and performance expectations generated by new
user devices and services in the next decade. The evolution towards
"ultra-massive MIMO (UM-MIMO)" is not only about adding more antennas but will
also uncover new propagation and hardware phenomena that can only be treated by
jointly utilizing insights from the communication, electromagnetic (EM), and
circuit theory areas. This article offers a comprehensive overview of the key
benefits of the UM-MIMO technology and the associated challenges. It explores
massive multiplexing facilitated by radiative near-field effects, characterizes
the spatial degrees-of-freedom, and practical channel estimation schemes
tailored for massive arrays. Moreover, we provide a tutorial on EM theory and
circuit theory, and how it is used to obtain physically consistent antenna and
channel models. Subsequently, the article describes different ways to implement
massive and dense antenna arrays, and how to co-design antennas with signal
processing. The main open research challenges are identified at the end.
