In order for educators to plan for thinking skills in the curriculum, what is meant by thinking must first be determined. Drawing from current research, this report provides working definitions of thinking skills and practical models to explain the working relationships among different levels and different kinds of thought processes. These definitions and models form a three level taxonomy: (1) Cognition--the essential skills (causation, transformations, relationships, classification, and qualification) and their relationships with complex processes (problem solving, decision-making, critical thinking, and creative thinking); (2) Metacognition--the skills of learner awareness of his/her own thinking (monitoring task performance and selecting and understanding the appropriate strategy; and (3) Epistemic Cognition--the skills for understanding the limits of knowing and the nature of problems that can be addressed by the thinker. With an understanding of this taxonomy of thinking, educators can examine the variety of materials and programs for enhancing thinking instruction in the classroom. It is also important to consider the role of subject matter in instruction emphasizing thinking and how student achievement in thinking abilities is to be assessed. (BS)