ABSTRACT This study aims to investigate the planning of bilingual teaching by in-service teachers in elementary and junior high schools in Taiwan. Through individual interviews and a questionnaire survey, the author examines (1) the principles that bilingual teachers (including content teachers and English teachers) integrate into bilingual lesson planning; (2) how bilingual teachers’ integration of these principles differs considering the types of teachers, levels of schools, and bilingual teaching experiences; and (3) challenges facing teachers in planning bilingual teaching. The findings indicate that when planning bilingual lessons, the teachers frequently considered the principles regarding the dimensions of content, language, teaching methods, teaching activities, materials, assessment, and class arrangement. On the other hand, teachers’ integration of the principles differed between teacher groups, and teachers encountered various challenges when planning bilingual teaching. Based on the findings, suggestions are made for bilingual lesson planning and teacher professional development.