There is considerable debate both in Australia and internationally about the best way for schools to deliver the thinking skills required for university study and professional work life -- skills that are often referred to by the term "critical thinking". This article reviews the literature on the effective teaching of critical thinking and looks at one example of a secondary school subject aimed at developing critical thinking skills in its graduates, namely the "Theory of Knowledge" (ToK) subject in the International Baccalaureate Diploma Program (DP). ToK, a core element of the DP, has no set curriculum. Instead, ToK requires students to think deeply about knowledge problems and to formulate coherent views on the nature of knowledge. This article reports on an Australian study into the teaching and learning of ToK and presents associated outcomes in cohorts of both secondary and tertiary students.