The evolution of the disk controllers that interface disk drive storage devices and subsystems with their hosts is reviewed. With magnetic disk storage density increasing by over five orders of magnitude in 37 years, the nature of the controller and its functions has undergone a significant change. In the 1960s, introduction of the IBM 360 and 370 systems led to architectures oriented to mainframe systems. In the 1980s, the personal computer brought into being the single-board controller most frequently attached to a single hard disk drive. In the 1990s, the functions that are being included within the drive are radically changing the attachment of a drive to a system.>