  With the standardization of 5G, commercial millimeter wave (mmWave)
communications has become a reality despite all the concerns about the
unfavorable propagation characteristics of these frequencies. Even though the
5G systems are still being rolled out, it is argued that their gigabits per
second rates may fall short in supporting many emerging applications, such as
3D gaming and extended reality. Such applications will require several hundreds
of gigabits per second to several terabits per second data rates with low
latency and high reliability, which are expected to be the design goals of the
next generation 6G communications systems. Given the potential of terahertz
(THz) communications systems to provide such data rates over short distances,
they are widely regarded to be the next frontier for the wireless
communications research. The primary goal of this chapter is to equip readers
with sufficient background about the mmWave and THz bands so that they are able
to both appreciate the necessity of using these bands for commercial
communications in the current wireless landscape and to reason the key design
considerations for the communications systems operating in these bands. Towards
this goal, this chapter provides a unified treatment of these bands with
particular emphasis on their propagation characteristics, channel models,
design and implementation considerations, and potential applications to 6G
wireless. A brief summary of the current standardization activities related to
the use of these bands for commercial communications applications is also
provided.
