At least half of states administer or are developing kindergarten entry assessments. In fall 2017 the Illinois State Board of Education began requiring teachers to report data on every child's skills at kindergarten entry using the Kindergarten Individual Development Survey. State and local stakeholders have asked for more information on the reliability and validity of the survey and on the gaps in children's skills at school entry. This study analyzed the psychometric properties of the 14 required items on the survey after its first statewide administration. It examined average skills and the variation in skill levels at kindergarten entry, as well as their differences across child subgroups and school poverty levels. And it interviewed teachers and principals about barriers in administering the survey and suggestions for improvement. The study found that the survey measures two developmental domains: learning and social skills, and academic knowledge and skills. Measures of these domains are psychometrically reliable and valid. Nearly 9 in 10 children (88 percent) had a score below the scale's midpoint for the learning and social skills domain, and 85 percent had a score below the scale's midpoint for the academic knowledge and skills domain. Children's skills differed across subgroups. The percentage of children in a school who were eligible for the national school lunch program was negatively associated with academic knowledge and skills at kindergarten entry, even after child-level eligibility for the program was controlled for. Teachers and principals reported multiple challenges in administering the survey--including difficulties observing all skills for every child, choosing between adjacent rating categories, and entering data into the online portal--and had several suggestions for improvement. [For the appendixes, see ED599358; for the brief, see ED599364; and for the snapshot, see ED599366.]