This chapter aims at unveiling the tactic knowledge by investigating reflection through collaborative lesson planning by some English language teachers in Vietnamese high schools. Reflective teaching has long been regarded as an important part of teacher professional development. The lesson planning sessions took place within two high school settings in a suburban area in Central Vietnam. These schools used a series of English language textbooks prescribed by the Vietnam Ministry of Education and Training (MOET). The lesson planning sessions provides substantial and rich data to understand teacher's intentions and their underlying rationales. The data provides a general impression that the teachers tended to retain certain types of tasks and activities in lessons and made decisions to change others based on their collective reflection of their personal and contextual constraints. The chapter discusses the use of lesson planning in capturing teaching reflection-for-actions, and raises implications and concerns regarding future use of the tool to investigate teacher's practices and reflective thinking.