  Terahertz (THz) band communications are envisioned as a key technology for 6G
and Beyond. As a fundamental wireless infrastructure, THz communication can
boost abundant promising applications. In 2014, our team published two
comprehensive roadmaps for the development and progress of THz communication
networks [1], [2], which helped the research community to start research on
this subject afterwards. The topic of THz communications became very important
and appealing to the research community due to 6G wireless systems design and
development in recent years. Many papers are getting published covering
different aspects of wireless systems using the THz band. With this paper, our
aim is looking back to the last decade and revisiting the old problems and
pointing out what has been achieved in the research community so far.
Furthermore, in this paper, open challenges and new research directions still
to be investigated for the THz band communication systems are presented, by
covering diverse topics ranging from devices, channel behavior, communication
and networking, to physical testbeds and demonstration systems. The key aspects
presented in this paper will enable THz communications as a pillar of 6G and
Beyond wireless systems in the next decade.
