Grounded in sociocultural notions of teaching and learning, this qualitative case study examines the practices of 10th-grade English as a Second Language (ESL) and English Language Arts (ELA) co-teachers to promote the academic language and literacy development of their shared students, including English learners (ELs), never-ELs, and fluent English proficient students (FEPs). Additionally, this study aims to increase co-teacher collaboration and asset-based perspectives of ELs. The data was collected in three phases of the co-teaching lesson cycle: lesson planning, lesson implementation, and co-teacher reflection. Data included audio-recorded interviews and planning sessions, artifact collection, video-recorded lesson observations, video logs, field notes, audio-recorded reflection discussions, and transcription. Data analysis indicated a support model of co-teaching, in which the ESL teacher supported the ELA teacher. Moreover, dominant notions of language and literacy development dominated classroom interactions. Findings suggest that while co-teaching can be a powerful means of teacher collaboration and innovative teaching practices to support high school ELs, the success of co-teaching partnerships is impacted by situated factors, which are examined and described in the study. [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.]