1.Evolution
MRP- I was a planning tool important in the 1970s.As an isolated unit，it has the disadvantage that，because it is an open system there are no feedback loops between inputs and outputs[1]. The basic MRP- I system thus evolved considerably into so-called MRP- II systems，or Material Resource Planning，and beyond this to business applications software systems that manage a company’s entire business.These are referred to as Business Resource Planning(BRP)or Enterprise Resource Planning(ERP)systems.
2.Feedback
In the MRP- II type of system，elements of the business plan are relayed through to the sales plan to the production plan，and then to the master production schedule.In these modules are feedback loops that provide information so that previous plans can be adjusted if resources are insufficient.The master production schedule then provides data for the material requirements plan，the capacity plan of the facility，purchasing requirements and shop floor activity.As earlier，there are feedback loops that make it possible to adjust，or propose modifications to，earlier plans if resources are insufficient [2]，Thus the whole system is a closed loop.This scheme is illustrated in Fig. 10-4.
MRP-II systems require that every employee，including the operator，analyst，quality inspector，sales person，purchasing agent，planning staff and even account staff，be thoroughly and strictly disciplined about entering current datum into the system.Managers can calculate the requirement of every part or subassembly，week by week，and track in advance possible delays or material shortages.People in inventory control can then plan the release dates and meet promised deliveries.At the business level，MRP-II verifies the marketing and production plans so that the availability of resources can be determined.Then，by combining the business plan with production goals，it considers what is in inventory，the time it takes for vendors to deliver，the cash flow and that dates when the product can be delivered to the customer.
3.Commercial ERP systems
Commercially，there are numerous complex ERP-type business applications software systems for aiding management in either manufacturing or service-related activities.The software systems are generally compiled in series of modules，each one covering particular functional elements of the firm such as：
· sales;
· manufacturing;
· accounting;
· inventory management;
· distribution;
· transportation;
· payroll management;
· project management.
Modules can either be stand-alone products or be combined with other modules to give a complete integrated system and are usually able to operate on Microsoft，UNIX or IBM AS / 400 systems.Fig. 10-5 gives a general presentation of what companies can offer.