The GreenFeed automated emissions monitoring (GF) system can be used to estimate CH4 emissions (g/d) from individual animals and has been available since about 2010 (Garnett 2012). The GF system is manufactured by C-Lock Inc. (Rapid City, SD, USA; https://www.c-lockinc.com [accessed 13/08/2020]) C-Lock also provides support services for maintaining and managing the GF system, including services to set up the GF system, provide training to use the system, monitor the system when in use to ensure proper function, data storage and processing, and provide parts and consumables (e.g. filters, standard gases). In view of the information already available, this chapter focuses mainly on what the operator or researcher needs to consider when implementing the GF system in animal experiments, including: setting up the GF system in different research environments; operator inputs into the GF system; animal adaptation; animal visitation to the system; trial duration; and other experimental factors of concern.