Although anarchism was introduced to Spain during the last quarter of the nineteenth century, it was not until the First World War that it attained the proportions of a mass movement. Following a brief review of the ideological development of anarchism up to the time of the Second Republic (1931–6), this chapter examines two competing tendencies that dominated the movement during the 1930s, namely, the syndicalists (treintistas) and the ultra-revolutionaries (faístas). Because it was during the Spanish Revolution and Civil War between July 1936 and March 1939 that anarcho-syndicalism experienced the apogee of its development in Spain, the central focus of this chapter will be on those aspects of anarchist wartime activities that illuminate both the strengths and the weaknesses of Spanish libertarianism.