An essay on the relative influence exercised by the First International (the Spanish Regional Federation of the IWA), the Second International, French revolutionary syndicalism (the CGT and the Bourses du Travail), and Italian syndicalism (the USI), respectively, on the origin and development of Spanish anarcho-syndicalism, with extensive discussion of the internal debates that took place between 1880 and 1920 in the European anarchist milieu on the general strike and the nature and purpose of trade unionism.