Research, education, and extension investments, while usually necessary, are often insufficient alone to bring knowledge, technologies, and services that enable farmers and entrepreneurs to innovate. Efforts to strengthen research systems and increase the availability of knowledge have not increased innovation or the use of knowledge in agriculture at the pace or the scale required by the intensifying and proliferating challenges confronting agriculture. Agricultural Innovation Systems: An Investment Sourcebook contributes to the identification, design, and implementation of the investments, approaches, and complementary interventions most likely to strengthen agricultural innovation systems (AIS) and to promote innovation and equitable growth. The Sourcebook provides a menu of tools and operational guidance, as well as good practice lessons, to illustrate approaches to designing, investing in, and improving these systems. Managing the ability of agriculture to meet rising global demand and to respond to the changes and opportunities will require good policy, sustained investments, and innovation - not business as usual. Experience indicates that aside from a strong capacity in R&D, the ability to innovate is often related to collective action and coordination, exchange of knowledge among diverse actors, incentives and resources available to form partnerships and develop business, and an enabling environment. While consensus is developing about what is meant by 'innovation' and 'innovation system,' no detailed blueprint exists for making agricultural innovation happen at a given time, in a given place, for a given result. That said, the AIS approach, which looks at these multiple conditions and relationships that promote innovation in agriculture in specific contexts, has moved from a concept to a subdiscipline with principles of analysis and action. Drawing on approaches that have been tested at different scales in different settings, this Sourcebook emphasizes the lessons learned, benefits and impacts, implementation issues, and prospects for replicating or expanding successful practices. The Sourcebook reflects the experiences and evolving understanding of numerous individuals and organizations concerned with agricultural innovation, including the World Bank. It targets the key operational staff who design and implement lending projects in international and regional development agencies and national governments, as well as the practitioners who design thematic programs and technical assistance packages. The Sourcebook can also be an important resource for the research community and nongovernmental organizations.