A decade ago, the audience for a discussion on “scaling” international development initiatives could fit comfortably in a telephone booth. But times have changed. Today, virtually every serious conversation about development includes attention to strategies for achieving and sustaining results at scale; and there is an emerging body of literature, tools, case examples, and communities of practice from which to learn.

This change reflects growing concern over the seemingly endless array of projects that fail to reach large numbers of the poor or to sustain outcomes over time, and the mismatch between the magnitude of many of the world’s most serious problems and the numerous but relatively small donor resources arrayed against those problems.

Despite this growing recognition, major challenges remain, including the need for fundamental changes in established policies, procedures, and priorities of donor agencies, national governments, researchers, and program implementers. Most specifically, these changes imply a reexamination of the current preoccupation with technological innovation and “pilot projects,” and a fundamental rethinking of ways to ensure that donor investments are more likely to catalyze lasting, systemic change. This reorientation requires (1) designing interventions with scale in mind and with clear scaling strategies; (2) assessing and addressing obstacles to scalability; and (3) actively managing the pathway to scale. The agriculture sector has been slower than some other sectors, most notably health, to incorporate these changes.

In an effort to close this gap, Purdue University organized a major conference in September 2018 on *Innovations in Agriculture: Scaling Up to Reach Millions*, in partnership with the African Development Bank. This Sourcebook is informed and inspired by that conference and is intended as an easy-to-use reference targeting a broad and diverse audience drawn from host governments, research institutions, and academic, business, policy, and donor communities concerned with leveraging agricultural innovation to meet the needs of the world’s poor.

The Sourcebook is divided into chapters addressing the following issues: designing with scale in mind; assessing scalability; using commercial markets to drive scaling; financing the transition to scale; creating an enabling environment for scale; tailoring metrics, monitoring, and evaluation to support sustainable outcomes at scale; and the critical role of intermediary and donor organizations. It is designed to be a stand-alone source of guidance, tips, and examples, and to provide links to additional resources for readers wishing for more detail.

Also summarized in the Sourcebook are many of the growing array of on-the-ground cases, donor practices, and analytic tools that can help to inform future efforts to scale agricultural interventions and outcomes.

The Sourcebook ends with a call to action and highlights the following conclusions:

• Delivery at scale is not a gigantic project or a series of projects. We need to plan for millions, not thousands; for uncontrolled, not controlled, settings; for generations, not for five years; and for addressing, not working around, political and market realities.
• This requires narrowing the gap between macro goals and micro interventions by linking the language and logic of projects to the language and logic of development effectiveness.
• Development assistance can help, but it will not solve the problem. Only markets and governments can; and commercial markets should normally be the default setting. Short-term interventions (“projects”) and subsidies can make big and positive differences, especially in reaching underserved smallholders and small and medium enterprises (SMEs), but only if used strategically. They can also introduce major distortions.
• It is essential to view agriculture as a business, not a social sector; to treat farmers as businesses and customers, not as beneficiaries; and to focus more attention on the full value chain, on finance, on incentives, on the intermediation needed to bring innovation to scale, and on the enabling environment.
• New partnerships and multi-stakeholder initiatives are essential for tackling scaling challenges.
• Initiatives must go beyond being “policy takers” and play a much more proactive role in facilitating policy change that can be a scaling force multiplier.
• There is rarely a straight line or a short journey from research and innovation to validation and rollout. Adaptive management is an essential ingredient in all successful scaling efforts.
• The actions of research institutions, nongovernmental organizations (NGOs), and other implementing partners are shaped by donor policies, metrics, and procedures. Systemic change, therefore, needs to give special attention to changes in the funding priorities, internal incentives, and operational procedures of those donor institutions.