A growing understanding of the importance of children's earliest years has led to an increasing desire to measure early childhood development (ECD) outcomes. There are now nearly 150 tools for measuring ECD outcomes internationally, which can make it challenging to choose an appropriate measurement tool for a given measurement effort. This document guides the user through the why, who, what, and how questions that must be considered prior to selecting tools for measuring ECD outcomes. Users should document their responses at each step to collate the information needed to identify and select an appropriate ECD measurement tool. [This guidance note was funded by the World Bank's Early Learning Partnership (ELP).]