Working questions are critical issues for the sustainabity of livestock farming systems (LFS) and for their adaptive capacities. Society and market chain pressure into new manners of doing that modify the tasks distribution and the priorities beetween tasks, for example, to delay hay making for floristic biodiversity preservation, or to fill up in time subsidies formulars or traceability papers. At the same time, increasing labour productivity is a general trend as well as the deep changes in the labour force composition (less family workers, more associations between farmers or wage-earning workers). Diversification and off farm activities also constraint the time avaible for livestock activities. More than ever, characterizing and evaluating a LFS (its rooms for manoeuvre), designing innovative ones that have to be integrated into real" farms, require 1) to consider farmers as technico-economical decisions makers but also as work organizers and workers themselves, 2) models that make intelligible the interactions between livestock and land management practices, workforce and the combination of activities of farmers. In this short paper, we present the main traits of a model Atelage (Madelrieux et al. 2006) that aims at characterizing and qualifying work organization in livestock farms.