In conclusion, by revealing strongly-bound excitonic behavior in the caesium titanium halide vacancy-ordered perovskites (Cs 2 TiX 6 ), we reconcile longstanding discrepancies between theoretical predictions and experimental measurements for this material class. While previous theoretical studies have found semi-local DFT to yield band gaps matching experiment, we show this to be the result of fortuitous error cancellation with qualitatively incorrect absorption spectra and relative band gaps for Cs 2 SnX 6 vs Cs 2 TiX 6 (X = I, Br).