The modernization of firms frequently involves the introduction of just-in-time production, considered as an efficient means to reduce costs and manage work and workers. But this organizational reform usually takes place within a framework of contradictory managerial strategies. Workers are given additional responsibilities without significant autonomy. The objective is to involve them in a system which remains prescriptive and based on a Taylorian logic. The authors draw upon a monograph on small and medium-sized enterprises to analyse the impact of this complex evolution, both on workers and on the overall performance of firms.