To work with a system, users have to be able to control the system and assess the state of the system. An HMI (Human Machine Interface) delivers information from machine to people, which enables the user to control, monitor, and record the system. HMIs can display critical real time data to the operator and in many cases allow adjustments to the system while it is running. In its simplest form, HMIs are set up for one-to-one direct interface with a single device, while others are set up to be part of a network with communication amongst several devices.

HMIs come in a variety of sizes and types and can be a fixed simple single screen to a multi-page programmable display. Some HMIs have keypads, some have touchscreens while others have both. Some HMIs are multi-color and some are monochrome; Some HMIs are set up manually while others can be configured off line. Some HMIs have a modem providing the ability to view, configure or edit the HMI over the Internet.