The number of English learners (ELs) entering middle schools continues to increase, and middle school content teachers providing EL instruction face a considerable challenge. The problem that was studied in this research was the middle school content teachers' insufficient understanding of ELs' cultural background, needs, and interests. The literature review indicated middle school content teachers receive training or workshops based on the assigned subject taught and less on accommodating ELs. The purpose of this qualitative case study was to explore middle school mainstream teachers' preparations, perceptions, and experiences encountered while teaching ELs in one Maryland school district. Vygotsky's (1978) sociocultural theory and the concept of zone of proximal development framed the study. The research questions used to clarify the problem focused on the factors affecting ELs' education such as teachers' preparation, perceptions, and experiences while teaching ELs in the mainstream classroom. Fifteen middle school content teachers took part in the study and answered the online questionnaire and interview questions in detail with interviews conducted one on one. Criterion sampling was used in the identification of the 15 teacher volunteers who completed online teacher questionnaires and one on one interviews. The data from two research instruments were cleaned, prepared, and analyzed, resulting in emergent themes. Findings are presented using figures and tables and data that showed the need for relevant professional development and in-service training for content teachers teaching ELs in the mainstream classroom. The findings suggested the need for greater collaboration among all stakeholders to support content teachers in more effectively instructing 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/en-US/products/dissertations/individuals.shtml.]