Conservation payments lead to improvement in environmental quality only if farmers and ranchers who receive them adopt conservation practices that would not have been adopted without the payment. When a voluntary payment causes a change in practice(s) that lead(s) to improved environmental quality, these changes are "additional." We estimate this "additionality" for a number of common conservation practices that are frequently supported by existing conservation programs. We find that the level of additionality varies by practice and that additionality is high for structural and vegeta-tive practices while the risk of nonadditionality appears to be higher for management practices. While the risk of nonadditionality cannot be completely eliminated, it can be reduced. We discuss a number of approaches to managing nonadditionality in both conservation programs and environmental offset programs.