From World War II to the early 70s, a very large number of surveys on Catholic observance were conducted in France. With great patience and tenacity, Canon Fernand Boulard (1898-1977), who had inspired many of thèse surveys, collected the data thus obtained and coded them. The présent authors were provided with Boulard's data, made them comparable and processed them with the help of computer techniques. The many maps, statistical tables and often innovative graphs enable them to illustrate the distribution (in over 3,000 rural cantons and two hundred cities and urban areas) of the various types of observants, according to sex, âge and occupation. This unique work also points out the géographie similarities and différences between Catholicism and France's economy, population and politics.