An alarm clock (or sometimes just an alarm clock) is a clock designed to alert individuals or groups of individuals at a given time. The main function of these clocks is to wake people up to sleep or nap at night. They are also sometimes used for other reminders. Most use sound; some use light or vibration. Some sensors have sensors to identify when a person is in a light sleep state, so as to avoid waking a sleeping person, even if the person has enough sleep, which can also lead to fatigue. To stop the sound or light, press the button or handle on the clock. If left unattended long enough, most clocks will automatically stop the alarm. The classic analog alarm has an extra pointer or an embedded dial to specify when to activate the alarm. Alarm clocks can also be found on mobile phones and watches.

The traditional mechanical alarm clock has one or two clocks, which ring through the main clocks, which provide power for the gears to push the hammer back and forth between two clocks or the inside of a single clock. In some models, the back cover of the clock itself acts as a clock. In the electric bell type alarm clock, the electric bell is knocked by the electromagnetic circuit and the armature, and the armature can repeatedly open and close the circuit.

The digital alarm clock may make other noises. The simple battery powered alarm clock makes a buzz or beep to wake up the sleepers, while the novel alarm clock can say, laugh, sing or play the sounds of nature.

Some alarm clock radios can be set to start playing at a specified time, which is called clock radio. Some alarm clocks can set multiple alarm clocks, which is very useful for couples with different wake-up schedules. Progressive alarm clocks are still very new in the market, and can have different alarms at different times (see next generation alarm clocks), or even play the music of your choice. Most modern TVs, mobile phones and digital watches have an alarm clock that turns on or makes a sound at a specific time.