This applied dissertation was designed to describe the research-based practices used by middle school teachers of multilingual or ELL students in the subject area courses and the teachers' self-efficacy to implement those research-based practices while teaching. Also, this study aimed to describe the impact of professional development regarding research-based teaching practices on the implementation of these practices and to compare the teachers' practices, their perceptions of self-efficacy and the impact of professional development had on their teaching. This study followed a quantitative methodology with a survey design. A non-probabilistic sampling was used for the selection of the participants. The sample consisted of 63 teachers from a public middle school in Florida. The sample included teachers of Language Arts, Mathematics, Science and Social Studies. A total of 52 teachers participated in this study. Four instruments were administered to collect the data to answer the research questions. The findings evidenced that in relation to research-based practices, the teachers used some of them more frequently than other practices in their classes. Regarding the teachers' self-efficacy, it was found teachers showed more self-efficacy in some practices than in others. In relation to the implementation of these teaching practices and the impact of the professional development on their teaching practices, the teachers perceived that some components were more effective than others. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]