The present study examined the amount, type, and quality of Tier 1 and supplemental reading instruction provided to fourth-grade struggling readers and whether specific elements of instruction predicted growth in reading skills. 110 students identified as having reading difficulty, receiving school-based reading instruction in 22 classrooms located in two states/sites, participated. Reading instruction was observed and coded for instructional dimensions, including reading components, grouping, and quality. Reading comprehension and vocabulary were the most prevalent components of instruction, while limited time was allocated to word-level reading skills. Several significant differences in time allocated to overall instruction and components of instruction during Tier 1 were noted between sites. Overall, there were few unique Tier 1 instructional predictors of student achievement at the end of the year. Students receiving supplemental reading instruction outperformed those students receiving only Tier 1 on measures of oral reading fluency. Implications for instruction and future research are discussed. [This paper was published in "Reading and Writing Quarterly" v33 p395-411 2017 (EJ1155981).]