This Instruction document describes the measurement of Green Area Index (GAI) and Aboveground Biomass (AGB) at ICOS ecosystem stations in mires. The focus of these Instructions is skewed towards rather sparsely vegetated fen and bog mires, as found in boreal and subarctic Europe and with vegetation primarily made up of mosses, herbaceous vascular plants, and dwarf shrubs. Only measurements on mire vegetation comprising one or more of these three plant functional types (PFTs) are hence explained here. If the target area includes other PFTs that should be measured, such as trees in stands, a measurement methodology and sampling design for these PFTs can be discussed and agreed upon with ETC. More generally, the application of this protocol can for each individual station be adapted and fine-tuned through discussion and agreement between station PI and ETC. The document starts with describing the spatial organisation of the measurements at each Continuous Measurements plot (CP) and then step-by-step the methods, measurements, and calculations to be done. Then is given the list of all the parameters that have to be submitted to the ETC’s database, including the correct formatting and the way to submit them. The document ends with a section on processing, which is for internal use of the ETC and describes the processing applied to the single variable/info/sample submitted.