Recent federal policy changes have placed new requirements on states to demonstrate and document their efforts to improve access to high-quality early care and education (ECE), and have made clear the urgent need for a shared understanding of this concept. This guidebook was developed to address the need for developing a common understanding and approach to measuring access. This Guidebook is intended to support states' efforts to assess the reach and effectiveness of their policy initiatives aimed at expanding ECE access. The Guidebook provides information in four sections: (1) Clarifying and Defining Access; (2) Describing the Indicators of Access; (3) Measuring the Indicators of Access; and (4) Identifying ECE Access Datasets and Sources. The goal of this Guidebook is to spark dialogue among policymakers, researchers and advocates about the importance of conceptualizing ECE access as a multi-dimensional concept and to propose innovative strategies for measuring and tracking progress toward improving access.