The purpose of the present study was to develop an instrument to measure reading during in-school independent reading (ISIR). Procedures to establish validity and reliability of the instrument included videotaping and observing students during ISIR, gathering feedback from literacy experts, establishing interrater reliability, crosschecking instruments, and minimizing observer bias. A factor analysis revealed items loaded onto four factors: attention to text, outward indicators, physical interactions with text, and spontaneous sharing of content. Further analysis indicated that involvement during ISIR was highly correlated with items associated with attention to text and the readers' responses. This article provides an overview of the development and initial field-testing of the instrument with recommendations for future use and development.