MICHAEL E. MARTINEZ is an associate professor in the Department of Education at the University of California, Irvine. THE DISCOVERY and theoretical elaboration of metacognition constitute a major breakthrough in recent decades of cognitive research. While it is always best to begin with definitions, a concept as complex as metacognition demands that our initial definition be provisional. This definition will become more detailed and complete as our understanding grows. Many teachers would describe metacognition, quite acceptably, as about But I would propose a more precise definition: metacognition is the monitoring and control of thought. In this article, I seek to clarify the concept of metacognition and to present ideas about how metacognitive ability can be taught. Metacognition serves many diverse functions, as does language. Consider what the Austrian philosopher Ludwig Wittgenstein had to say about language: Think of the tools in a tool-box: There is a hammer, pliers, a saw, a screw-driver, a rule, a glue-pot, glue, nails, and screws. The functions of words are as diverse as the functions of these objects. (1) The toolbox is an equally apt metaphor for metacognition. To give a sense of the breadth of metacognitive functioning, I have identified three major categories of metacognition: metamemory and metacomprehension, problem solving, and critical thinking. While this simple taxonomy does not exhaust the many types of metacognition, it does communicate the broad role of the process in important cognitive endeavors. Metamemory and metacomprehension. I have grouped these two concepts together because both refer to an understanding of one's own knowledge state. To grasp metamemory, think about being asked if you are able to name the planets of our solar system in order of their distance from the sun. Now, you either can or cannot do so. But your answer to the question might be yes or no in either case. That is, you can be accurate or inaccurate in your appraisal of your own knowledge. Is this accuracy important? Yes, it is. Research has shown that this self-insight has predictive power for subsequent learning. (2) Metacomprehension is a similar concept with similar importance to learning. Comprehension during reading or listening can be either good or poor. But independent of learners' true comprehension is whether they realize that their comprehension is good or poor. This appraisal can sometimes be very inaccurate. You can easily imagine a student reading a textbook page and not understanding it--and moreover not realizing that he does not understand it. Believing that one understands when one does not is a serious and common error. But even more serious is when students don't even consider whether they comprehend but simply take notes or read mechanically. Problem solving. This almost exclusively human pursuit is a significant category of cognitive activity. Problem solving is exercised daily, often continuously, especially in a complex society where following established rules and procedures is not enough to succeed. Problem solving can be defined simply as the pursuit of a goal when the path to that goal is uncertain. In other words, it's what you do when you don't know what you're doing. Scientific research is a form of problem solving; so is teaching. Neither of these enterprises is algorithmic. Both involve continuously generating possibilities, weighing those options, exploring subsets of options, and evaluating the results. Herbert Simon used the metaphor of finding your way through a maze to describe this process. Problem solving certainly involves cognition. But more is required: constantly stepping back mentally to appraise and rework plans by asking such questions as, What am I trying to accomplish? What are the most promising pathways? Is my strategy working? Critical thinking. Like problem solving, critical thinking encompasses a lot of what human beings do--or at least potentially can do. …