This paper is a part in a bigger research project of secondary school teachers' cognitions of the textbooks and the teaching methodology (Task-based Language Teaching). The purpose of this is to mention some theoretical issues in researches of teacher cognition, and then to propose the suggestions for the research project of secondary school teacher cognition. Firstly, the paper presents various concepts of teachers' cognitions. Secondly, this paper gives out educators' viewpoints of the importances of teachers' cognition of teaching methodology. Nextly, some researches in teacher cognitions of learning and teaching are discussed and reviewed. Lastly, the research in secondary school teacher cognition of textbooks and teaching methodology (TBLT) in Vietnam is suggested.