Within recent years French historians have turned increasingly to police archives as a vast untapped source for the study of labor and socialist movements. However diligent the labor press was in reporting movement activities, it could never outdo the French political police in its saturation coverage of meetings, congresses and demonstrations. During the second half of the nineteenth century the hub of the political police was the Paris Prefecture of Police, which had literally hundreds of paid informants reporting on political and social circles, high and low, not only in Paris, but in the provinces and other countries as well. Fire destroyed nearly all of the valuable archives of the Prefecture prior to 1871. Beginning then with the post-Commune period, which is particularly rich in documentation, the archives are presently open to the public with some restrictions through 1940. A visit to the Archives of the Prefecture of Police of Paris, located in the Section Police Judiciaire, 36 quai des Orfèvres, has become de rigueur for students of French and international labor movements.