Research Findings: There is a growing need for accurate and efficient measures of classroom quality in early childhood education and care (ECEC) settings. Observational measures are costly, as their administration generally takes 3-5 hr per classroom. This article outlines the process of development and preliminary concurrent validity testing of the Assessment for Quality Improvement (AQI), a new measure of global quality. The AQI is a classroom-level measure of structural and process quality. It consists of 24 items on a 5-point scale designed for use in ECEC infant and toddler classrooms. At between 60 and 90 min per room, the AQI is a relatively efficient measure. Item response theory modeling was used to ensure logical and coherent ordering of subitems. Exploratory factor analysis supported the use of the AQI total score and the Interactions section as a stand-alone measure. Correlations between the Infant and Toddler versions of the AQI and the Infant/Toddler Environment Rating Scale-Revised were moderate, providing preliminary support for the concurrent validity of both versions. Practice or Policy: Our results suggest that the AQI is a promising, efficient measure of global quality in infant and toddler ECEC environments. This may be especially relevant for Quality Rating and Improvement Systems, for which the observational component is a major cost driver. (author abstract)