The Early Development Instrument (EDI), a teacher-completed measure of children's school readiness at entry to Grade 1, was designed to provide communities with an informative, inexpensive and psychometrically sound tool to assess outcomes of early development as reflected in children's school readiness. Its psychometric properties at individual level were evaluated in two studies. Five a priori domains -physical health and well-being, social competence, emotional maturity, language and communication, and cognitive development and general knowledge -were tested in a factor analysis of data on over 16,000 kindergarten children. The factor analyses upheld the first three domains, but revealed the need to develop two new ones, resulting in the final version of the EDI consisting of: physical health and well-being, social competence, emotional maturity, language and cognitive development, communication skills and general knowledge domains. These final domains showed good reliability levels, comparable with other instruments. A separate study (N = 82) demonstrated consistent agreements in parent-teacher, interrater reliabilities, concurrent validity, and convergent validity. These results establish the EDI as a psychometrically adequate indicator of child well-being at school entry.