Safety has always been the primary objective of air traffic control, which must never be compromised. Automation removes some existing sources of human error but introduces new ones. Some of these can be diagnosed beforehand, and human-machine relationships can be used to highlight their occurrence and to minimise their consequences. If the system can operate in more than one mode, this can lead to errors of mode misrecognition. Built-in diagnostics can aid error prevention, and help to reveal how far the effects of any error could extend. Much more is now known about the diversity of the origins and effects of human error, but there are still practical difficulties in integrating this knowledge into system design.