The aim of this study is to identify differences in opinions on the evaluation of ELT teachers' books between pre-service and in-service teachers. In literature, it has been argued that the development of teachers' books for EFL teachers can be assisted by the results of a needs analysis. A total of 65 pre-service and 50 in-service secondary school teachers from South Korea took part in a survey. There was a high level of agreement for the 22 criteria-items that asked for the evaluative opinions between two groups as shown through the correlation r = 0.85. However, there were statistically different opinions. The pre-service teachers stressed the importance of clear objectives for each lesson, but the in-service teachers emphasized the need for the inclusion of process tests in teachers' books. Interestingly, there were also different opinions on the use of a teacher's book. The results from semi-structured interviews revealed that the former plan to use the teacher's book for their classroom teaching in the future whereas the latter responded that their own notes would be somewhat more useful than teachers' books. To conclude, it can be proposed that greater emphasis should be placed on gauging preferences of certain groups of teachers in specific EFL contexts for the development of teachers' books.