Old cadastres are largely used as sources in a good many geohistorical publications. Although often reduced to their planimetric aspect alone, they form the kernel of a substantial documentary output. The constraints on the making of these documents (registers and maps), but also the constraints we place on them when using them for ends other than their initial fiscal purpose, must be highlighted so researchers can guard against making improper use of them. Cadastres form the basis of many geohistorical reference frameworks and must be subject to critical editing that is as thorough and true as historians’ editing of texts. The specific features of cadastral sources as documents invite us, to do this, to take up and use the tools of diplomatics and text genetics and ultimately to suggest a few methodological pointers.