Despite extensive and widespread knowledge of the advantages of agricultural anaerobic digestion (AD), adoption of the technology has not been uniform across the globe. What explains this uneven adoption across countries? Policy and empirical evidence from five case study countries - Germany, Denmark, Netherlands, Austria and the United States - indicate that rather than comparative technological advantage or abundance in feedstock availability, differences in adoption was the outcome of differences in policy incentives, notably the feed-in tariff, a finding that offers empirical support to the threshold model of adoption. The stable financial support of a feed-in tariff provided to investors in agricultural AD, particularly in Germany, led to wide adoption. The evidence also suggests that differences in the enactment of the feed-in tariff was influenced by energy security concerns for policy leaders, but by learning-by-doing in terms of policy implementation and lower operating costs for policy followers. (C) 2016 Elsevier Ltd. All rights reserved.