The traditional approach to learning is to define the objectives (the gap between the knowledge a person has and the knowledge the person needs to perform the task), establish the regimen for practice, and provide feedback. Learning procedures and factual data is seen as adding more information and skills to the person's storehouse of knowledge. However, this storehouse metaphor is poorly suited for cognitive skill, and does not address the differing learning needs of novices and experts. Teaching cognitive skills requires the diagnosis of the problem in terms of flaws in existing mental models, not gaps in knowledge. It requires learning objectives that are linked to the person's current mental models. It requires practice regimens that may have to result in "unlearning" that enables the person to abandon the current, flawed mental models. It requires feedback that promotes sensemaking. We propose a Cognitive Transformation Theory to guide the development of cognitive skills. Finally, we present several strategies that might be useful in overcoming barriers to understanding and to revising mental models.