Heat pumps are a key component of Switzerland's energy transition. Project 66 “Optimal Heat Pump Design and Operation” of the IEA Heat Pumping Technology Collaboration Programme investigates internationally used heat pump concepts in terms of development, design, operation, and control. The aim is to derive best practice recommendations for heat pump types. The project combines field measurements, laboratory and literature data, promotes international exchange, and provides practical insights for manufacturers, planners, and policymakers. Optimized systems can increase efficiency, reduce electricity consumption, and contribute to the achievement of national energy and climate targets. Other participants come from the United States, Germany, Spain, and Sweden. An additional work package is dedicated to predicting part-load behavior, which is currently of great importance. In addition, the Swiss support group will be regularly informed about the project's progress in a Project 66 newsletter, and the Sweet DeCarbCH information channels will be used to disseminate the project results effectively and quickly.