Research on language assessment knowledge (LAK) of teachers has focused on two major topics: identifying the LAK needs of teachers and developing appropriate LAK tests. Although the prior research findings significantly contributed to our understanding of the parameters of LAK, they were mostly quantitative and did not provide much information about EFL teachers' perceptions and applications of their LAK in a direct and face-to-face situation. Therefore, this qualitative study was designed to shed light on some key issues related to teachers' LAK using semi-structured interviews. The issues included EFL teachers' perception of their LAK and their utilization of LAK in their teaching. The participants were 11 teachers with a high level of LAK and 10 teachers with a low level of LAK determined by their performance on a LAK test. The interviews were recorded, transcribed, and content analyzed. The findings did not reveal significant differences in the responses provided by the two groups of teachers. Further, to investigate the extent of teachers' application of LAK in classroom contexts, some of the tests made by the participating teachers were collected and content analyzed. The results showed that teachers with high LAK wrote longer tests with more varied sections and tasks. Finally, no meaningful relationship was found between the teachers' level of LAK and their students' performance on classroom achievement tests. The findings imply that the language assessment field needs more research on multiple dimensions of LAK.