Superheavy Elements: Beyond the 7th Period in the Periodic Table

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.