Major art heists have repeatedly exposed weaknesses in museum security while creating mysteries that can last for decades. The 1911 theft of the Mona Lisa made Leonardo’s portrait internationally famous. The 1990 Gardner Museum burglary remains unsolved, while the 2019 Green Vault raid demonstrated the continuing threat to historic treasures. Stolen artworks are difficult to sell because their identities are widely known & legitimate buyers demand reliable provenance. Recovering them therefore requires international cooperation, detailed records, strong security & persistent investigation.