Air-source heat pumps have historically shown declining capacity and efficiency at low outdoor temperatures—exactly when heating demand is highest in cold climates. The U.S. Department of Energy's Residential Cold Climate Heat Pump Challenge, completed in 2024, tested whether manufacturers could produce equipment meeting DOE's targets for rated capacity and efficiency at temperatures as low as −15 °F (approximately −26 °C). Products that qualified demonstrate that the engineering constraints are not insurmountable. The Northeast Energy Efficiency Partnerships maintains a product list updated with field-measured performance data. ENERGY STAR's cold-climate heat pump criteria provide a standardised benchmark. What the challenge results do not establish: that every installation using a qualifying product will eliminate backup heat; that performance at the test point translates to whole-season average performance; or that installation quality, duct design, controls configuration, and building shell are secondary factors. Cold-climate heat pump performance in practice is a whole-system result. Sizing matters: oversized equipment short-cycles and fails to dehumidify. Undersized equipment runs backup resistance heat more than planned. Installer familiarity with cold-climate commissioning varies considerably. DOE's Building Technologies Office provides ongoing updates. NEEP's product database allows comparison of measured heating capacity and coefficient of performance across outdoor temperature ranges. Editorial caution: Qualification at a DOE test condition is not evidence that every installation will eliminate backup heat. Report performance at the relevant outdoor temperature, distinguish rated from seasonal average efficiency, and do not present manufacturer heating-capacity figures as the whole story.