There is no doubt that, of all the theoretical events in chess, arising from the Interzonal Tournament of Gothenburg 1955, it is the birth of the "Poisoned Pawn Variation", creation of Paul Keres, which has undoubtedly had the greatest significance until today. The present work would not exist today if the author had not had the stimulus of having received many words of praise on the occasion of the publication of the first studies on this very complicated line of play. Throughout more than 50 years, in one way or another, he has felt the need to update the body of the study, incorporating the interesting novelties produced between versions, up to the present one, which in the sequence corresponds to the fourth. More than 50 years working on the same theme, when the Poisoned Pawn Variation is completing 68 years of existence.