The purpose of this study is to identify the types of professional knowledge, which are activated by pre-service teachers of English, when they design a lesson plan. This study utilized the qualitative research methodology. As research subjects, 60 pre-service EFL teachers from three universities located in Chile were considered. Non-participant observation and a semi-structured interview with stimulated recall were used as techniques to generate data. Content analysis was employed to process the data by means of ATLAS.ti software. The findings suggest that the participants mobilize different types of professional knowledge when they design a lesson plan. These involve content knowledge, general pedagogical knowledge, pedagogical content knowledge, curricular knowledge, knowledge of learners, and knowledge of educational contexts. It is recommended that EFL pre-service education fosters prospective teachers’ reflection on their teaching practices so that they can reshape, in a progressive and permanent way, their professional knowledge connected to successful teaching practices.