Establishing a common understanding of ECE access, and how to measure it across different types of early learning settings, is essential for state and local policymakers responsible for improving access. A common understanding of access allows policymakers, administrators, and researchers to communicate clearly about this important concept. A common set of measurable indicators of ECE access allow for accurate longitudinal and cross-state or intrastate comparisons, as well. The ECE Access Guidebook was developed to address the need for developing a common understanding and approach to measuring access. Ultimately, 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: Clarifying and Defining Access; Describing the Indicators of Access; Measuring the Indicators of Access; and Identifying ECE Access Datasets and Sources. (author abstract)