The goal of this study was to examine oral word reading fluency from a developmental perspective in a longitudinal study of students from second grade to sixth grade. The sample consisted of native English speaking students that took part in a large longitudinal study. Participants were assessed on cognitive and literacy measures such as working memory, phonological awareness, rapid automatized naming and syntactic awareness--oral cloze. Two main research questions were examined: first, what relationships will be found between the cognitive, literacy and linguistic measures, and which of them simultaneously predict oral reading fluency, in each age group? And second, which cognitive and literacy measures in second grade predict word reading fluency in sixth grade? Results show that cognitive and literacy measures contribute differently to word reading fluency skill across the different grades, while the only strong predictor across all age groups, was the phonological awareness. Finally, taking together past and previous findings, a proposed definition of fluency from a developmental perspective is suggested, based on the results of the study that show in a clear manner, that reading fluency, its contributors and its predictors, change respectively to the reading phase obtained in each grade.