Air source heat pumps are space-conditioning appliances that use vapor-compression technology to provide both heating and cooling to buildings. In recent years, several companies have introduced cold-climate air source heat pumps that can provide space heating at outdoor temperatures below zero degrees Fahrenheit. The Cold Climate Housing Research Center, the Alaska Center for Energy and Power, and the UAF Bristol Bay Campus have been collaboration on studying the performance of air source heat pumps in Alaska. This presentation will talk about the underlying physics as well as technological limitations and the resulting performance of air source heat pumps in cold climates.