This "Building Capacity for Change" "playbook" highlights practices and strategies that proved to be effective in supporting the rollout and implementation of certain programs under the New York State Department of Health's First 1,000 Days on Medicaid Initiative, a series of evidence-based reforms aimed at improving health and social outcomes for expectant parents, young children, and their families. Recognizing that advancing the health and well-being of the state's youngest citizens requires a well-coordinated comprehensive strategy, the Rockefeller Institute of Government partnered with the Department of Health to develop and incorporate a framework of collective impact--bringing the strengths and resources of organizations across program and policy sectors together to work toward a shared goal--for selected reforms that were being piloted in target communities across the state. This playbook is designed as a guide for Medicaid providers and other early childhood health practitioners, local community organizations, and state policymakers on how collective impact strategies can be used to promote cross-sector coordination and align resources in ways that advance maternal and early childhood outcomes. Wisdom and evidence gained from working with providers, partner organizations, and families in the field as these Medicaid reforms were rolled out are discussed, and supporting data and project outcomes resulting from efforts embracing a strategic collective impact approach are presented. The playbook is organized into four main parts: background on New York's "First 1,000 Days on Medicaid Initiative" to offer context for the depth and breadth of this bold reform effort; a discussion of the collective impact framework and strategies for implementing it in local areas where social service reforms are piloted; and two sections describing project activities in local areas piloting two "First 1,000 Days on Medicaid" reforms.