1 INTRODUCTION
The last few years have seen rapid progress on phased array feeds (PAFs) for synthesis imaging as well as large, single-dish telescopes. Developments include cryogenically cooled front end electronics, antenna element design for wide bandwidth and active impedance matching for maximum sensitivity, digital back end development, and image processing and calibration algorithms. This paper reports on several key areas of collaborative work at Brigham Young University (BYU), National Radio Astronomy Observatory (NRAO), and the National Astronomical Observatories of China (NAOC) towards high sensitivity, single-dish PAFs.