This article analyses the construct validity of the International Baccalaureate Diploma Programme's Theory of Knowledge course in the light of claims that it is a course in critical thinking. After discussion around critical thinking--what it is and why it is valuable educationally--the article analyses the extent to which the course aims, assessment objectives and assessment instruments emphasise critical thinking. The article concludes with suggestions for improvement in the writing of the Theory of Knowledge guide so that it might place more emphasis on certain strands of critical thinking that are currently not developed in its structure.