This chapter analyses the extent and the nature of public interest and involvement in the Dreyfus Affair, in geographical, political, and social terms. Something can be gleaned in answer to the first question about the geography of opinion, from the police reports on disturbances during the Affair, on Dreyfusard and anti-Dreyfusard meetings, and on the distribution of sections of the various Leagues created or recreated on one side or the other. The chapter then considers the involvement or non-involvement in the Affair of existing political groups, studying in particular the behaviour of the more independent organizations of the Left and the participation of groupings of the Extreme Left. The police reports yield evidence of antisemitism and “nationalism” among workers, and it is known that the anti-Dreyfusard Leagues tried, with partial success, to recruit them. Meanwhile, in the involvement of the middle and upper classes in the Affair, one finds that several groups, professional and confessional, stand out: students and university teachers, the officer corps, Catholic clergy and laity, Protestants and Jews. The last three categories are not, of course, exclusively middle- or upper-class, but militancy among them does seem to have been mainly a phenomenon of the elite.