  What is the heaviest element? In order to address this question, the elements
up to $Z=118$ have been synthesized by now by using heavy-ion fusion reactions.
This has completed the 7th period in the periodic table of elements, and new
attempts have been commenced in aiming at syntheses of the elements in the 8th
period. In this article, we review the current status and future challenges in
the research field of superheavy elements, putting some emphasis on
perspectives from the nuclear reaction theory.
