Abstract: The Cosmic Microwave Background (CMB) provides a unique window on the physics of the early Universe and measurements of CMB polarization offer the potential of a definitive test of inflation. Many of the advances in the field of CMB science have been enabled by advances in detector technology. Ground-based CMB experiments have seen order of magnitude increases in detector count with each subsequent stage with current stage-3 experiments fielding ~10K detectors. In this talk I will provide an overview of CMB detector development at Argonne. For over a decade we have been developing transition edge sensors (TES) through our collaboration with the South Pole Telescope, resulting in the delivery of detectors for the SPTpol and SPT-3G experiments. Current efforts are focused on developing detectors for the CMB-S4 experiment, which is expected to have ~ 300K detectors. I will discuss issues of detector design, fabrication, and testing necessary to realize the large detector counts of these experiments.