This study investigates what Korean English teachers perceive as the elements that constitute their expertise and identifies the need enhance their expertise. In order to derive research results, a survey was conducted targeting elementary and middle school teachers. Fifty-eight elementary, middle, and high school English teachers participated in the survey, and six experts collaborated on compiling the results . The results show that teachers strongly recognize the ability to implement customized classes as an element of expertise; in contrast, the areas which do not reflect the situation and context of their classroom are perceived as less important. Additionally, four implications for English teacher development were derived through focus group interview. First, the teacher development program, which currently relies on teacher autonomy, requires a certain amount of mandatory training. Second, the concept of training needs to be expanded. Third, in order to revitalize the professional learning community system, it is necessary to impose minimum obligations on members. Fourth, teachers should proactively participate in research, work together, share the results with their colleagues, and have those colleagues conduct follow-up research, thereby impacting their classes positively.