The cooperation between theoretical and experimental investigations broke the record for the superconducting critical temperature T<inf>c</inf>in hydrogen sulfide at a high pressure at the end of 2014. Surprisingly, the material improved its highest T<inf>c</inf>by more than 30 K and showed conventional superconductivity, which can be explained by the Bardeen–Cooper–Schrieffer theory. Recent experimental works have gradually clarified the instability of the H<inf>2</inf>S molecule and the pathway to the high-T<inf>c</inf>phase with a three-dimensional conductive structure unlike high-T<inf>c</inf>superconductors thus far. In this review, the present progress on a sulfur hydride system is reported.