The Measuring Early Learning Quality and Outcomes (MELQO) initiative began in 2014 in anticipation of a new global emphasis on early childhood development (ECD). Led by UNESCO, the World Bank, the Center for Universal Education at the Brookings Institution, and UNICEF, the initiative aims to promote feasible, accurate and useful measurement of children's development and learning at the start of primary school, and of the quality of their pre-primary learning environments. This report is intended as an overview of the MELQO initiative, outlining the overall purpose of the effort, the module development process and rationale, and the content of the modules themselves--MODEL, focused on measuring child development and learning, and MELE, focused on measuring early learning environments. It is organized into three main sections: (1) Background and rationale: This section provides a discussion of why measurement of ECD is important and how the MELQO initiative grew out of the current dialogue about ECD and equity in the SDGs; (2) Building the MELQO modules: This section provides an introduction to the MELQO modules, including a description of their purpose and content, and a brief overview of how they were developed; and (3) Using the MELQO modules: This section presents the processes for using and adapting MELQO within the country context, including key considerations for integrating MELQO into existing country systems and building capacity for measurement efforts. The report is accompanied by two technical manuals which detail the constructs and items for each module at the end of MELQO Phase 1 and describe the process for adapting and using them within countries. [This report represents the collaborative work of the Measuring Early Learning Quality and Outcomes (MELQO) core team, technical advisory groups and steering committee, as well as the individuals and organizations consulted in countries where the modules were piloted.]