This chapter pays tribute to the social psychologist Henri Tajfel, famous for formulating social identity theory. The chapter asks why Tajfel did not apply his theory to the event that brought him into academic life: the Holocaust. Clues can be found in a short introduction which he wrote for new edition of Peretz Bernstein’s book of anti-Semitism. Bernstein had proposed a general theory of group relations to explain anti-Semitism. What Tajfel, and also Bernstein, wrote after the war about Bernstein’s pre-war book is revealing. The sort of language which might have been appropriate for writing about anti-Semitism in the 1920s had become wholly inappropriate after the Holocaust. The singularity of the Holocaust would be misplaced if were seen merely as an example of ‘prejudice’. Significantly, Tajfel avoided using his own ‘social identity theory’ to explain Nazism. His praise of Bernstein expresses an understanding and depth of feeling that show the limitations of theory.