An air conditioner and a method for controlling an air conditioner are provided that efficiently inject a refrigerant into a compressor. The air conditioner may include a compressor to compress a refrigerant; a condenser to condense the refrigerant compressed at the compressor; an evaporator to evaporate the refrigerant condensed at the condenser; and an injection module to selectively or simultaneously inject some or a portion of the refrigerant flowing from the condenser to the evaporator into a high pressure side and a low pressure side of the compressor.