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We outline Otal’s proof of marked length spectrum rigidity for negatively curved surfaces. We omit all technical details, and refer the interested reader to the original [Ota90] or the course notes [Wil] for details, and to [Cro90] for different approach. (Actually the course notes [Wil] combine the approaches in [Ota90,Cro90].) The author thanks Amie Wilkinson for explaining this proof to him....