A set of techniques which uses bills of material, inventory data and the master production schedule to calculate requirements for items. It makes recommendations to release replenishment orders for material. 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 as primarily a scheduling technique, i.e., a method for establishing and maintaining valid due dates on orders. It is the foundation for Closed-loop MRP.