This study is of professional development of secondary teachers seeking an English as Second Language (ESL) endorsement. Participants are secondary teachers of a major urban metropolitan school district with over 70% student population that is identified as speakers of a language other than English (LOTE). The study analyzes teachers' understanding of knowledge, skills and dispositions associated with teaching English Learners (ELs) after these teachers have completed a long term, coherent professional development program designed for urban secondary teachers of one school district. In seeking a determination, the study utilizes two guiding research questions. The first research question addresses what mainstream teachers say about their knowledge, skills and dispositions relative to teaching ELs. The second focuses on a more generalized understanding of what mainstream teachers say is important to understand about EL students. In order to interpret findings, the study utilizes two theoretical frameworks, Knowledge-for-Practice (Cochran-Smith & Lytle, 1999) and Cultural Relevant Teaching (Villegas & Lucas, 2002b). The primary data instrument is an e-survey, which includes open-ended and Likert questions. Data analysis includes an SPSS analysis for descriptive statistics, measures of internal reliability and Spearman correlation analysis, as well as constant comparison method (Glasser & Straus, 1967; Straus & Corbin, 1994) of data from responses to open-ended questions. The findings suggest that teacher participants understand that supporting EL students' first Language facilitates connections to prior learning in their first language to school content. Respondents identify that scaffolding, heterogeneous grouping, and allowing of first language use among students are ways that foster learning of English while learning content. In terms of language perspectives on the use of English-only or English plus ELs' first language in classroom teaching, some respondents support English-only instruction for learning English and content while others identify the importance of first language support while learning English and content. Supporting ELs' cultural background is deemed important by respondents as a way of promoting EL student academic success. Respondents also identify supporting ELs' academic success through EL advocacy among fellow teachers as means to educate and guide teachers who are unfamiliar with teaching ELs. [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.]