Validation of a classroom observation measure for use with English Learners (ELs) in grade 1 was the focus of this study. Fourteen teachers were observed during reading and language arts instruction with an instrument used to generate ratings of instructional quality on a number of dimensions. In these classrooms we assessed the reading performance of all ELs, as well as a sample of native English speakers, at the beginning and end of the school year. Technical characteristics of the observation measure and the reading growth of ELs are described. The major finding was that observers were able to rate instructional practice in a way that predicted classroom reading growth. Correlations were on the order of 0.6 to 0.75, indicating a relatively strong relation between ratings of instructional practice and student reading growth. We interpret the relation between classroom instruction and ELs' reading growth using a framework of measurement validity developed by Messick that proposes an integrated conception of validity. We use the framework to discuss data analysis and interpretation and the potential implications of these interpretations for instruction and for professional development in early reading with ELs.