The most critical and urgent reason a smoke detector alarms is because of a fire. If this is the case, quickly evacuate the premises and call the police and fire department. Fire is a serious issue that can claim lives and destroy homes if not taken care of immediately.
Many smoke detectors run using a battery. When the battery gets low, your smoke detector may alarm to get your attention, warning you its battery is running dry. Smoke detector's should be tested frequently, and batteries should be changed at least once a year. Remember: many hard-wired smoke detectors have a backup battery that must be changed as well.
If you live in a complex or work in a building with a hard-wired fire alarm system, your smoke detector could be triggered by a drill. Fire drills are used to maintain structure and safe practices in case of an actual fire. Talk to your landlord or the head of the building for more details about evacuation practices for that particular location.
Sometimes a smoke detector will sound if too close to a smoke producing appliance. Ovens and heating units sometimes generate small amounts of smoke that cause a smoke detector to alarm. When installing smoke detectors, keep them a fair distance from these appliances. Use vents and open windows when cooking to prevent unnecessary sound offs.
If you've tested your smoke alarm, changed the battery and tried everything else under the sun, perhaps it's time for a replacement. Get a new smoke detector as soon as possible to ensure your home stays safe.