Developing a successful early childhood integrated data system (ECIDS)--a longitudinal data system incorporating data from early childhood programs--involves a diverse set of stakeholders actively guiding the project through a collaborative decisionmaking process. ECIDS stakeholders include early childhood programs, children and their families, state advisory councils, academic researchers, legislators, federal policymakers, and any other group with a stake in the project's outcomes. Effective stakeholder engagement helps ensure the ECIDS provides the information these groups need to make important decisions about early childhood--from families deciding whether to enroll their child in a specific program to federal policymakers deciding how to modify program requirements and funding. When ECIDS stakeholders understand the system's benefits and take ownership of its management, they become vital advocates for its future growth and sustainability. This publication explores the diverse roles of stakeholders in developing and implementing an ECIDS. It covers strategies for promoting positive stakeholder engagement and lessons learned from Wisconsin and Illinois for involving stakeholders in all stages of an ECIDS project. It also highlights the differences between stakeholder engagement and data governance activities. (author abstract)