The Mass. Global Warming Solutions Act Implementation Advisory Committee- presented a preliminary proposal in 2020 to convert ONE MILLION housing units to heat pump systems (mostly from fuel oil but some from natural gas) between 2020 and 2030. The intention is to meet the legally mandated goal of >80% GHG reduction by 2050. Efforts to use heat pumps in the existing housing stock has revealed a wide range of challenges. Some of these challenges have resulted in disapointing operational costs; comfort issues; and less-than-expectd GHG Reductions. Approaches to addressing these challenges are discussed using case studies with data on real world delivered performance.