Notwithstanding the constant debate in the scientific and policy literature on the precise meaning of school readiness, research consistently demonstrates a wide variation between groups of children resulting in a gap at school entry. Recently, the teacher-completed Early Development Instrument (EDI), a new measure of children's school readiness in 5 developmental areas, was developed, tested, and implemented in Canada. EDI results confirmed the existence of a school entry gap. In this article, we explore factors in 5 areas of risk: socioeconomic status, family structure, child health, parent health, and parent involvement in literacy development. In a series of logistic regressions, we demonstrate that variables in all 5 areas, as well as age and gender, contribute to the gap. Child's suboptimal health, male gender, and coming from a family with low income contribute most strongly to the vulnerability at school entry. As the purpose of a tool like the EDI is primarily to assist in population-level reporting on children's school readiness, the results of our study provide additional and much-needed evidence on the instrument's sensitivity at the individual level, thus paving the way for its use in interpreting children's school readiness in the context of their lives and the communities in which they live. (Contains 6 tables.)