We study in this article mathematics teachers' "documentation work": looking for resources, selecting/designing mathematical tasks, planning their succession, managing available artifacts, etc. We consider that this documentation work is at the core of teachers' professional activity and professional development. We introduce a distinction between available "resources" and "documents" developed by teachers through a "documentational genesis" process, in a perspective inspired by the "instrumental approach." Throughout their documentation work, teachers develop "documentation systems," and the digitizing of resources entails evolutions of these systems. The approach we propose aims at seizing these evolutions, and more generally at studying teachers' professional change.