The well-known trademark gained ground towards the end of the XX century and beginnings of the XXI; At the time of the new century, the joint recommendation concerning the provisions on the protection of well-known marks within WIPO, prior and concurrent to this, is being followed by many countries elaborating more on this figure in their domestic rules. In many legal texts it is indicated, as in the recommendation already mentioned, that in order to determine notoriety, the competent authority will take into consideration any circumstance from which it can be inferred. However, invoking the notoriety of a trademark for purposes of protection in administrative and judicial proceedings, entails for the claimant the burden of a complex requirement of evidence, even in the face of obvious or notorious facts. Speakers will present examples in comparative law on how well-known trademark protection is provided, as well as experiences, legislation and court decisions on the elements that “prove them as such”, including the concept of “notorious fact”. The discussion will lead us to conclude how notorious is the well-known trademark in the legislation and in the judgment of the courts.