IBM System z, or earlier IBM eServer zSeries, is a brand name designated by IBM to all its mainframe computers.

zSeries servers are used by IBM customers for business-critical installations in medium and large organisations which need very high availability, where scheduled and unscheduled downtime costs are high, and at traditional mainframe shops such as banks and insurance companies which already have mainframe applications at the center of their business processes. For such organisations which have to consider a very high price for system failures and service outages, zSeries machines provide a lower total cost of ownership than other platforms, especially when running a variety of business-critical applications concurrently (so-called mixed workload). Overall, mainframes like the zSeries are mostly used in government, financial services, retail, and manufacturing industries.

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