The purpose of this study was to discover how teachers perceive the effectiveness of a constructivist-based sheltered instruction program in the mainstream classroom. This study also examined how the sheltered instruction program implementation has impacted teachers' perceptions of their own teaching practices, as well as how they view the adequacy of professional development in sheltered instruction. Additionally, this case study tracked a cohort of 59 students, who attended three elementary schools in a suburban district where constructivist-based sheltered instruction strategies have been consistently implemented. The goal of this component of the research was to discover whether there was a significant difference between English language learners' reading and math scores before and after the constructivist-based sheltered instruction implementation, as measured by the Oklahoma Core Curriculum Tests. Based on a survey completed by 56 voluntary participants, teachers indicated that consistent implementation of sheltered instruction in the mainstream classroom is effective in supporting English language learners. The 59 English language learners, whose reading and math scores were tracked for the year before, during, and after the implementation of the sheltered instruction program, scored significantly higher on their reading and math tests for the year during and after implementation of sheltered instruction than they had for the year before the implementation of the sheltered instruction program. Findings from the qualitative grounded theory showed that teachers perceived consistent implementation of sheltered instruction strategies as effective in providing comprehensible input for English language learners in the mainstream classroom.