This project has studied patterns of ‘inventive concentration’ in green technologies. The approach has been operationalised using patent data on fuel cells — potentially one of themost important ‘green’ technologies. Six measures were described and tested: the coefficient of variation; the Herfindhal index; the 4-firm and8-firm concentration ratios; the Lotka coefficient; and the Gini coefficient. Initially, the analysis focused on US firms but becomes comparative to include Japan, Germany, UK, France, Canada, Australia, Switzerland, Italy, Sweden, Netherlands, and Israel. This allows the level of agreement among the various measures to be assessed and the nations to be ranked in terms of the concentration of their fuel cell patent production. This sector is concentrated in all 12 nations with Canada (Sweden) exhibiting high (low) levels of concentration across all measures. These are discussed in the context of recently published international ratings of national innovative capacity along with directions for future research.