In this intervention study, teachers tried to implement four instructional principles derived from the literature on research-based, explicit reading comprehension instruction in their fifth-grade classrooms. The principles focused on relevant background knowledge, reading comprehension strategies, reading-group organization, and reading motivation. Results indicated that during a five-month intervention period, students in the intervention group increased their strategic competence and comprehension performance relative to controls. However, no effect was found on reading motivation. The overall pattern of results is explained in relation to the implementation quality of the four instructional principles, with implementation data indicating that the principles of reading-group organization and reading motivation were particularly difficult for the teachers to translate into classroom practice.