This document advice for countries on using remote-sensing and ground-based data to estimate greenhouse gas (GHG) emissions and removals associated with REDD+ activities, and recommendations on establishing national measurement, reporting and verification (MRV) systems. This is the first of a series of methods and guidelines documents developed by the Global Forest Observations Initiative (GFOI), aimed at estimating future carbon stocks to support countries in their efforts to build national forest monitoring systems, which will help ensure credible, comparable and transparent forest carbon assessments. GFOI aims to accomplish this through helping to: secure the long-term provision of satellite Earth observation data to countries; and build capacity to use such data in combination with ground-based observations to estimate and report on changes in forests and their carbon stocks.