During a long time Brittany was presented as a Catholic and rural region rooted on the right. However, the analysis of the relationships of political forces and their evolution during the first 20th century reveals a more complex and diversified reality. What about the right and the centres? What is the weight of left-wing parties? What is the impact of the two global conflicts on ongoing political changes? Attention was drawn to the slippages and reclassifications of elected representatives according to national issues (following the First World War and the Tours Congress, during the crisis of the 1930s and the Popular Front, under Occupation and Liberation). The intersection between the roots of political parties and the action of men (questioning traditional remarkable figures, permanent or renewed elected representatives, mobilisation of activists) with the strength of the various influence networks (Catholic, agricultural, trade union, secular) makes it possible to draw a picture of the political life of a region which retains a strong cultural identity, with departmental or intra-departmental differences, while being increasingly integrated into France. This book is intended to summarise the numerous political history works carried out in Brittany’s universities for some 30 years.