The set of industry-standard commands used to send instructions to a modem. Each command line must start with the two-character attention code AT (or at). Developed for modem use by Hayes Microcomputer Products. Modems that use the AT command set and a Hayes-patented modem-escape sequence are said to be Hayes compatible.

Your modem is always either in the command mode, or the on-line mode. The modem starts up in command mode when it is first switched on. Commands are only accepted by the modem when it is in command mode. Commands input when the modem is on-line, are treated as data, not as commands. Commands may be entered from the terminal mode of most communications software packages.