The researcher in this study investigated questions that explored (a) what are the frames of reference of mainstream teachers who instruct English learners in the mainstream classroom, (b) how the use of critical reflection and implementation of sheltered instruction observation protocol (SIOP) influence mainstream teachers' efficacy of instructional delivery, and (c) how do mainstream teachers construct meaning and encourage their professional growth through their use of critical reflection and sheltered instruction observation protocol. Using a case study methodology, the researcher sought to answer these questions using relative literature and theories that exist within the field of education. The goal of the research was to add to the body of literature regarding teachers' professional growth through the use of critical reflection and sheltered instruction observation protocol. The findings of the study indicated participants used strategies similar to SIOP-based strategies, although they had no previous knowledge or formal training in the SIOP method. The findings also indicated participants held similar frames of reference associated with instructing English learners (ELs) in the mainstream classroom. Participants indicated inconsistency in the understanding of critical reflection as a concept and how to use it as an instructional habit efficaciously. Collaborative working environments for teachers of ELs is necessary in the process of critical reflection. Mainstream teachers' professional growth can be supported by using rational discourse to learn and share quality teaching practices. Professional development can support the practice of critical reflection and sheltered instruction observation protocol among mainstream teachers of ELs to influence mainstream teachers' critical examination of instructional habits. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]