As possibly one of the most important purposes of the mathematics education is to raise autonomous students who can convey their thoughts and reasoning within the social context, mathematical communication skill which is an inseparable part of mathematics and mathematics education has a place in mathematics curricula. When considering the importance of mathematical communication both in the school life and daily life, it is apparently required to help improve these skills of students’. Teachers play a key role in achieving this objective. Hence, this study aims to examine teachers’ opinions on mathematical communication skills and development and indicators of this skill. Using the case study method, the research was conducted with 15 secondary school mathematics teachers through semi-structured interviews. The data obtained were analyzed with the content analysis method. It was observed in the study that the teachers can regard mathematics as a language, think that they should use the mathematical language efficiently and properly in the first place so students can acquire the mathematical communication skills; majority of them care about the development of this skill and arrange the learning settings in accordance with this skill; and the teachers prefer the written examination the most for assessing students’ progress in their mathematical communication skills. It was also discovered what the teachers emphasize the most is understanding the given mathematical expression (definitions, terms, operations, symbols, etc.) among the subskills which are the indicators of mathematical communication skills and that they accentuate understanding the symbols, explaining the symbols in written and orally and using the symbols properly Communication is a way for sharing thoughts and clarifyingcomprehension (National Council of Teachers of Mathematics [NCTM],2000). Each discipline has its idiosyncratic language in considerationthat conveyed information, thoughts and skills in every discipline mayvary. Mathematics is a universal language with its own symbols andterminology and significant relations between its concepts (Ministry ofNational Education [MoNE], 2013). Students add meaning to theirexperiences via language. Mathematical thinking starts developingnaturally among children in the pre-primary school period. Children make sense of their surroundings by the means of observations andcommunications in the social environment they are in (MoNE, 2017). Itis therefore impossible to isolate language in daily life and mathematicslearning. Students need to use their own language to convey what theyhave found while exploring mathematics and thinking mathematically toothers and to clarify their findings through their observations. Proper andeffective use of language encourages learning in mathematics. Playing acritical role in expressing mathematical ideas alongside mathematicalsymbols and drawings, language functions as a bridge in the transitionbetween abstract and concrete notations (NSW Department of SchoolEducation [NSWDoSE], 1989). In other words, mathematical language isan instrument used in mathematical communication, mathematicalthinking and the process of instructing the mathematical concepts(Jamison, 2000; Mercer & Sams, 2006).As possibly one of the most important purposes of the mathematicseducation is to raise autonomous students who can convey theirthoughts and reasoning within the social context (Pourdavood, Svec,Cowen & Genovese, 2005), mathematical communication skill (MCS)which is an inseparable part of mathematics and mathematics educationis emphasized significantly in mathematics curricula in Turkey and manyother countries (i.e. USA, UK, Canada [Ontario], Singapore and NewZealand) (MoNE, 2013, 2017; NCTM, 1989; NCTM, 2000; NSWDoSE,1989; Singapore Ministry of Education [SMoE], 2007; The NationalCurriculum for England [NCfE], 1999; The New Zealand Ministry ofEducation [NZMoE], 2009; The Ontario Ministry of Education [OMoE],2005).When considering the importance of mathematical communicationboth in the school life and daily life, it is apparently required to helpimprove these skills of students’. It is highlighted by several studies andinstitutions that teachers have a great responsibility for enabling thedevelopment of MCS among students. Cooke and Buchholz (2005) statethat teachers need to help students express their thoughts in a detailedway so that they can establish a connection between mathematics andlanguage and this connection can be reflected in the best way possible.In addition, they lay emphasis on the idea that such kind of interactionsenable students to clarify their own thoughts and develop their ownunderstanding while trying to understand to comprehend their worldsthrough communication. When students asked to express what they doand think of, this does not only clarify and improve their ownunderstanding but also help them convey their levels of understandingto teachers (NSWDoSE, 1989). Hence, teachers are liable to use severalmethods to enable students to communicate about mathematics andassess their progress (Thompson and Chappell, 2007).Even though it is highlighted in several studies and by severalinstitutions that teachers have great responsibility for the developmentof students’ MCS, it is remarkably observed in the literature that studiesare needed for the teacher attention to the skill and assessment ofstudents’ MCS and progress. In the light of these considerations, thisstudy aims to discover how secondary school mathematics teachersassess MCS, its importance and students’ progress in MCS and toexamine teacher opinions on the indicators of MCS.This research which used the case study method was conductedwith 15 volunteered mathematics teachers who are serving at secondaryschools. Semi-structured interviews were performed to collect data in therese