Student engagement has been shown to be essential to the development of research-based best practices for K-12 education. It has been defined and measured in numerous ways. The purpose of this research study was to develop a measure of online student engagement for grades 3 through 8 using a partial credit Rasch model and validate the measure using confirmatory factor analysis. The dataset for this research study comprised approximately 20,000 online students in grades 3 through 8 from five different online schools. Two random samples of 10,000 students each were drawn for the measure development process and the validation of the measures created. For this research study student engagement was defined as a three-component manifestation of cognitive engagement, affective engagement, and behavioral engagement, which are required to achieve success as measured by normalized state assessments. This research study used tracked online student behaviors as items. Online student behavior items were converted from continuous to categorical after assessing indicator strength and possible inverted U relationship with academic achievement. The measure development and item categorization processes resulted in an online cognitive engagement measure and an online behavioral engagement measure for grades 3 through 8, with each grade having its own measure. All measures were validated using the second random sample of students and all but two (grades 4 and 5) were further validated by confirmatory factor analysis to be two factor models. Future research will include measure development specifically for students receiving special education services, comparing measures developed using the original continuous items without categorization, identification of facilitators of online student engagement for grades 3 through 8 and further evaluation of the relationship between online student engagement and academic achievement. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]