I was a bit surprised to see this question, since this should be readily available information elsewhere, so I did a short search to see if there is a problem finding out the answer.  It turned out there is, in that there are a lot of incorrect or incomplete answers out there.  The best answer I came across was this:"All air conditioners try to freeze up, that is the goal of the refrigerant... to chill.It is the design of the system that prevents this freezing up from occurring.There are three basic reasons for air conditioner freeze ups:1) The two most common causes of insufficient air flow are dirty filters and under sized ducts. A dirty filter limits air flow which in turn allows the cooling coil’s temperature to drop below freezing. Replacing the filter, monthly, usually alleviates this problem. Under sized ducts will also cause the system to freezeBecause it also limits the amount of air flowing over the cooling coil. This results in lowering the coil temperature and allowing the humidity in the air to collect on the coil and freeze.2)Refrigerant charge.

The refrigerant system teeters on a delicate balance. If the system is not charged correctly, the system will not function properly. This malfunction may result in the poor cooling output and/or freezing up. Only a qualified, certified service technician can properly test and charge a refrigerant system.3) Outdoor temperature. If night time temperatures are too low, the system will not operate properly, and freeze up may occur. The outdoor unit does not function well in temperatures below 60 degrees F. A special “ambient temperature” control must be installed to allow the unit to run properly in cool temperatures, or you may open your windows and save energy."  (, Air Conditioning Freeze Up .)Too many of the answers given on the web assume it is low charge, when it most likely is poor air flow due to an obstruction, which can be the filter, the coil itself being dirty, blocked vents, closed or dampered vents, or incorrectly set fan speed.  It can also be a symptom of too much refrigerant charge.