A Programmable Logic Controller (or PLC) is a specialised digital controller that can control machines and processes. it monitors inputs, makes decisions, and controls outputs in order to automate machines and processes. A building automation system is a system that controls and monitors building services. These systems can be built up in several different ways. In this chapter a general building automation system for a building with complex requirements due to the activity, such as a hospital, will be described. Real systems usually have several of the features and components described here but not all of them. The Automation level includes all the advanced controllers that controls and regulates the Field level devices in real time.