Teacher talk is of noteworthy significance in mainstream education and several attempts have aimed at evaluating it. The existing tools, nevertheless, are only general frameworks and with no specific focus on the main functions of teacher talk in ELT contexts. The present study gives a comprehensive account of ELT teacher talk and aims at developing a teacher talk instrument whose items are specific to ELT classes. Accordingly, extensive analysis of the related literature and the researchers' developed concept of the main functions of teacher talk were used as the foundation to develop a preliminary theoretical model of ELT teacher talk. The proposed theoretical conceptualization was refined through observation of the talk of ELT practitioners as well as interviews with ELT teachers and teacher educators. Ultimately, the theoretical conceptualization led to the development and validation of an observational English Language Teacher Talk Functional Scale (TTFS). The scale was piloted and then administered through having the talk of 182 ELT teachers rated by ELT teacher educators for final validation. TTFS was validated through both exploratory and confirmatory factor analyses. The result indicated that the constructed items in the TTFS and the conceptualization of the construct of teacher talk were meaningful to teachers, teacher educators, and teacher observers and indicative of the effectiveness of teacher talk in ELT contexts. We hope that the present study results in a more worthwhile understanding of the functions of teacher talk and provides an efficacious assessment tool for reflective practice and constructive feedback to ELT teachers.