the debate whether esp teachers need to have knowledge of the specialized field they are teaching has been an issue in the literature over the last two decades. the aim of conducting this study was to explore iranian learners’ and instructors’ beliefs and attitudes concerning the role of specialized knowledge in teaching esp classes. a seventeen item questionnaire was administered to 400 esp learners, 40 language instructors at five universities in iran. the data were analyzed using frequency distribution and chi-square. the results revealed that a substantial number of respondents maintained that language instructors need to understand the technical and scientific use of english utilized in different academic books. correspondingly, they agreed that in esp classes the language instructor is not in the position of being the primary knower of the subject matter of the materials because language instructors have literary or language background. moreover, a good majority of respondents believed that language instructors code switch to l1 and translate the content into persian when they lack specialized knowledge. similarly, a good majority of respondents agreed that esp teaching requires special skills to deal with specialized content. a solid majority of respondents also agreed that efl instructors having a humanities background should become familiar with the rhetorical, discoursal, and communicative features of different texts. similarly, a good majority of the respondents reported that teaching esp courses requires more experience, additional training, extra effort, and a fresh commitment compared with being an instructor of general english. furthermore, the findings of this study also reveal that cooperation and collaboration with content specialists would help prospective and actual esp instructors become more confident and competent practitioners. through collaboration and cooperation they can also select appropriate content, methodology, and activities for esp courses.