National forest inventories are a primary source of data for national and large area assessments of sustainability and biodiversity and for international forest resource reporting. However, the ability of countries to produce compatible estimates using these data is impeded by the diversity of their national inventory definitions, sampling designs, plot configurations, measured variables, and measurement protocols. In addition, because the differing features of these inventories have historical, commercial, and environmental justification, prospects for standardizing inventories are minimal. The best current alternative is to harmonize estimates, a process that focuses on developing methods for producing compatibility despite the different inventory features. Action E43 (COST E43, 2008, Harmonization of national forest inventories in Europe: Techniques for common reporting, Available at online at www.metla.fi/eu/cost/e43/index.html; last accessed May 2008) of the European program, Cooperation in the Field of Scientific and Technical Research (COST), has focused on developing harmonization methods for European inventories in three areas: harmonized definitions, harmonized estimation procedures for carbon pools, and harmonized indicators for biodiversity assessments. The Forest Inventory and Analysis program of the US Forest Service has participated in COST Action E43 and has provided data for testing proposed harmonization methods. Although considerable progress has been made in developing methods to facilitate harmonization, considerable work still remains.