The proliferation of an expanding English second language community has led to an increasing number of English language learner (ELL) students enrolling within the California public schools system. This research study sought to understand how teacher self-efficacy affected ELL assessment outcomes and the role professional development had in developing teacher self-efficacy. A gap analysis of the chosen high school was conducted and three areas were identified as affecting teacher self-efficacy: skills, knowledge, and motivation (Clark & Estes, 2008). The quantitative data were derived from a convenience sampling of certificated high school teachers who voluntarily participated in a pre- and post- teacher efficacy scale-English language development (TES-ELD) survey. An intervention in the form of a professional development workshop was conducted with participating credentialed teachers at the school of choice for a comparison analysis between pre- and post-surveys. Qualitative data were derived from a semi-structured interview protocol representing a variety of teacher demographics. Results of the study revealed the workshop positively influenced teacher self-efficacy in the three domains identified: skills, knowledge, and motivation. The workshop also increased awareness of the importance teacher-ELL student rapport is to developing teacher self-efficacy. This research provides educational leaders and teachers some guidance as to the importance of structured professional development for building teacher self-efficacy and recommendations for developing a strong sense of teacher self-efficacy that prioritizes equitable ELL assessment practices, which will enable teachers to inspire and motivate their ELL students to achieve their goals. [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.]