Collaboration at many levels - between governments, non-governmental organizations, regional groups, and academic disciplines - will create the fabric from which a durable peace can be established in the post-9/11 era. Private foundations, even without the vast resources of government agencies, can play an important catalytic role in encouraging collaboration across the spectrum of peacebuilding. Transaction costs and barriers to negotiation can make collaboration difficult for foundations and grantees, but the potential gains from collaboration overcome the costs of time and the risks of uncertainty inherent in collaborative ventures. Foundations can be especially helpful in establishing collaborations linking theory and practice (especially in the area of evaluation); in bringing together grantees in particular geographical areas; and in helping disseminate best practices and lessons learned from peacebuilding experiences. Foundations can create an even stronger voice for peacebuilding if foundation leaders are willing to join together as effectively in the peace and security area as they do in areas such as the environment and public health.