This brief provides an overview of definitions of readiness, noting distinctions between individual readiness, organizational readiness, and system readiness. We also share examples of how ECE researchers are currently attempting to capture the dimensions of readiness, and factors that support or hinder readiness, through different data collection methods and standardized measurement tools. While not an exhaustive list of measurement options, the summary tables provided share resources that ECE researchers and policymakers may want to consider when developing future studies of quality improvement initiatives. We conclude this brief by considering several conceptual and measurement issues that might be considered when preparing to implement new QI initiatives in the ECE field. (author abstract)