There is a significant need for kindergarten entry assessments (KEA) that meet state education agency (SEA) requirements and are psychometrically sound measures of a broad range of school readiness domains such as language, literacy, math, science, executive function, and social-emotional skills. Research Findings: In this paper, we describe five phases of development, calibration, and launch of a new KEA for a large state. We developed and tested 14 English subtests. We describe how teacher input and SEA priorities and policies guided development of the test blueprint in phase one. We calibrated the measures across the state in phase two and established initial evidence of reliability and validity in phase three (n = 208). In phase four, we developed our technology platform, scoring, and student grouping tools to improve data utilization. Practice or Policy: We describe in phase five current delivery and implementation practices across the state and future work to improve and expand the measures along with a set of linked activities to help teachers use data to guide instruction. We discuss principles and methods the assessment developers utilized, as these perspectives may inform the development and usage of other KEAs.