Reading comprehension is an essential skill that all students must master for learning to happen. The complexity of academic texts requires research-based instruction in reading comprehension that goes beyond the basic literacy level. The tiered texts approach is an effective, systematic approach for teaching reading comprehension where a series of progressively challenging texts on the same topic is used with a variety of instructional strategies and activities to scaffold both content instruction and language learning to prepare English learners (ELs) and struggling readers to read a grade-level target text. This paper reports on an action research study that explores whether in-service teachers in a teacher-training program can improve their skills in teaching reading comprehension of complex texts by developing a tiered texts unit and supporting their students in using it. It provides an overview of the tiered texts approach, describes the use of an operational model for designing and planning a tiered texts unit to scaffold the reading of a difficult text, and illustrates the approach with a unit designed for a secondary English language arts class with ELs and struggling readers. It also describes the qualitative analysis of the trainee teachers' reflections on their experience designing and implementing the unit. The results of the action research study supported the usefulness of the tiered texts approach in helping teachers gain knowledge and skills for instruction and revealed potential challenges in implementing this approach in classroom teaching.