The mainframe product line introduced in 1990 by IBM (superseding System/370) that features ESA/390 architecture and operating systems, ES/9000 hardware (18 models introduced), ESCON fiber-optic channels, Sysplex multiprocessing and SystemView.

In 1994, IBM introduced its next generation of System/390 systems known today as Parallel Enterprise Servers or S/390s. These are SMP machines that contain single-chip CMOS CPUs and use less power and dissipate less heat than the bipolar-based ES/9000 models. In 1995, IBM introduced new models that provide up to 10-way SMP within the same machine. Up to 32 10-way systems can be hooked together providing a multiprocessing system with up to 320 CPUs.

As processing requirements increase, customers are migrating from the ES/9000s to clusters of the CMOS-based machines. In 2000, IBM changed the name of S/390 mainframes to "zSeries eServers" (see IBM server series). See Parallel/Sysplex.