Excited-state double-proton transfer (ESDPT) in the coplanar 7-azaindole dimer (7AI2) has been studied as a model base pair. Concerted and stepwise mechanisms were proposed for ESDPT in 7AI2. In the concerted mechanism, two protons transfer concertedly on the excited-state potential energy surfaces where a local minimum does not exist. On the other hand, an intermediate state exists in the stepwise mechanism. Therefore, after one single-proton transfer, the second single-proton transfers via the intermediate state. Numerous spectroscopic and theoretical studies were conducted to determine the mechanism of ESDPT of 7AI2. This review focuses on the ESDPT mechanisms in the gas phase. A prologue and epilogue of the concerted and stepwise mechanism controversy are presented. Spectroscopic experiments in the condensed phase and theoretical calculations related to the potential energy surfaces are important for deriving a final conclusion. In addition, some significant results are comparatively discussed.