An alarm device or system of alarm devices gives an audible or visual alarm signal about a problem or condition.

Alarm devices include:

• Burglar alarms, designed to warn of burglaries; this is often a silent alarm: the police or guards are warned without indication to the burglar, which increases the chances of catching him or her.

• Alarm clocks can produce an alarm at a given time

• Distributed control manufacturing systems or DCSs, found in nuclear power plants, refineries and chemical facilities also generate alarms to direct the operator's attention to an important event that he or she needs to address.

• Alarms in an operation and maintenance (O&M) monitoring system, which informs the bad working state of (a particular part of) the system under monitoring.
o first-out alarm

Alarms also need to be capable of directing the operator's attention to the most important problem that he or she needs to act upon, using a priority to indicate degree of importance or rank, for instance.

The most important benefit of a home security system is that it will often prevent a home from being broken into. Therefore the chances of a property owner having their valuable belongings stolen are greatly reduced.