Critical thinking comprises the mental processes, strategies, and representations people use to solve problems, make decisions, and learn new concepts. The study of critical thinking combines the educational, philosophical, and psychological traditions of thought. R. Ennis offers a philosophical taxonomy suggesting that critical thinking results from the interaction of a set of dispositions toward critical thinking with a set of abilities for critical thinking, while R. Sternberg's psychological taxonomy defines the skills involved in critical thinking to be of three kinds: metacomponents, performance components, and knowledge-acquisition components. Bloom's taxonomy of education puts knowledge at the lowest level, followed by comprehension, application, analysis, and synthesis, with evaluation at the highest level. Tests for measuring critical thinking also come from the philosophical and psychological areas. The Watson-Glaser Critical Thinking Appraisal, the Cornell Critical Thinking Test, and the New Jersey Test of Reasoning Skills are derived from the philosophical tradition. The Triarchic Test of Intellectual Skills is psychologically derived, but does not try to separate critical thinking from intelligence. Programs for training critical thinking include Copi's 1978 course in logic, Bransford and Stein's 1984 course called "The Ideal Problem Solver," Sternberg's 1986 "Understanding and Increasing Intelligence," and Whimbey and Lochhead's 1982 "Problem Solving and Comprehension." Educators current concern with critical thinking offers students a new chance for developing critical thinking skills, but training must be brought into all aspects of the classroom to be successful. (Tables outlining E.J. Gubbins's Matrix of Thinking Skills, and Sternberg's program for training intellectual skills are appended.) (SRT)