The purpose of this study was to determine the current status of secondary teachers' implementation of the sheltered instruction operational protocol (SIOP) model and its effect on Hispanic English language learners' (ELL) English language proficiency and academic achievement. In addition, this study sought to determine whether teachers perceive the SIOP model as an effective tool for instruction of high school ELL students to increase English language content and English language proficiency. This study employed qualitative and quantitative methodologies. Data were collected from four sources: Hispanic ELLs' English language proficiency scores, students' English Language Arts scores, an oral interview with participating teachers and teacher observations. Each teacher was observed at four points during the school year with the SIOP instrument. Quantitative data on student achievement were collected employing a pre-experimental, one-group pretest-post-test design. Qualitative data were collected using a time-series design. Findings revealed that on the two student assessment measures there were increases in English proficiency and English language arts achievement among the Hispanic ELLs. On the assessment of English language proficiency, the students of the teacher with the highest level of SIOP implementation made the highest gains; the students of the teacher with the second highest SIOP implementation level made the second highest gains; and students of the teacher with the lowest level of SIOP implementation made the smallest gains. These findings suggest that the higher the level of SIOP implementation, the greater the student academic achievement gains. The gains in academic achievement attributed to the proper implementation of the SIOP model can have an extensive impact on English language learners who have not previously experienced academic success. Teacher participants perceived the SIOP model as effective for delivery of content through sheltered instruction lessons for high school ELLs. The teachers agreed that the SIOP model's components provided a consistent structure for planning and delivery of their sheltered lessons.