In the original EOS proposal competition in 1989, Dr. Steve Running proposed and was selected as MODIS Science team member responsible for Leaf area index, evapotranspiration and photosynthesis/net primary production, then designated as MOD 15, 16 and 17. At the ATBD review for at-launch products in 1995, NASA decided to give MOD 15 LA I/FPAR to Dr. Ranga Myneni to provide a more theoretically based algorithm, and Dr. Running was directed to focus on MOD 17 PSN/NPP for the Terra atlaunch data product. MOD 16 ET was not dropped, but was deprioritized. At the EOS recompete in 2003 NASA selected another investigator to build a MOD 16 ET product but this investigation was not renewed in 2007. In the interim Dr. Running and the NTSG group had changed from an energy balance - surface resistance concept to a Penman-Monteith concept, and had greater success building a globally applicable algorithm. Since much of the processing paralleled our MOD 17 product, NTSG tested, then generated initial global ET datasets. In the 2010 renewal competition for the MODIS Science Team, Dr. Running reproposed MOD 16, based on the new algorithm and global ET datasets now developed, and published in refereed journals. Now, with selection of our 2010 renewal proposal complete, we offer the ATBD. This document represents our formal ATBD for establishing this algorithm and dataset as the official MOD 16 Evapotranspiration product