After a brief history of the proposals for the mechanism of the ATP synthase, the main experimental arguments for a rotational mechanism of catalysis are analyzed and on the basis of this analysis it is concluded that no evidence has been provided for rotation as an obligatory element of the catalytic mechanism. On the other hand, the experimental evidence in favor of a two-sites catalytic mechanism, derived from various approaches and not compatible with a three-sites rotary mechanism, appear to be very solid. Finally a brief characterization of the various nucleotide binding sites is provided and a suggestion is made how the enzyme has evolutionarily developed from a rotating machine into an asymmetrical device for energy conservation.