The effects of instruction in critical thinking are investigated, using instructional materials based on a theoretical model of critical thinking. The model emphasized the role of concepts and processes in responding to instances of valid and invalid reasoning, argument, and evidence. Instruction focused on eight cognitive operations: generalization, representation, inference, analogy, explanation, information source, justification, and attribution. Among the instructional materials were programmed student workbooks, each focusing on one kind of reasoning using valid and invalid examples. Two sets of booklets were developed. One set of workbooks contained instructional activities in a deductive sequence, and the other used an inductive sequence. Pretests, criterion-referenced achievement tests, student questionnaires, and teachers' manuals were also developed in accordance with the theoretical model. A group of 369 sixth graders in northern California were randomly assigned to work independently in the workbooks in daily, 40-minute periods for two weeks. Pre- and posttest data indicate that instruction based on the theoretical model yielded significant learning in the experimental group. Neither the inductive or deductive method was significantly more effective. Tables illustrate the model, concepts used in the booklets, and inductive and deductive instructional sequences. (Author/AV)