The first step in the evolution of ERP. This set of techniques uses bills of material, inventory data, and the Master Production Schedule to calculate requirements for materials. It makes recommendations to release replenishment orders. Further, since it is time phased, it makes recommendations to reschedule open orders when due dates and need dates are not in phase. Originally seen as merely a better way to order inventory, today it is thought of primarily as a priority planning technique (i.e., a method for establishing and maintaining valid due dates on orders)