Learning is knowledge-dependent; people use knowledge to construct new knowledge. Effective learning depends on the intentions, self-monitoring, elaborations, and representational constructions of the individual learner. The phenomenon of knowledge-dependent learning poses urgent questions for a theory of instruction. Students of cognition and instruction face the challenge of identifying for many domains the implicit principles that might play this constraining role in learning and devising ways to make school learning as sensitive to these principles as possible. Implicitly held mental models can also contradict new ideas being taught and interfere with learning. Learning mathematics, physical and social sciences requires an understanding of such theoretical systems. Instruction must provide information for learners' knowledge construction processes. Traditional instructional theory assumes that knowledge and skill can be analyzed into component parts that function in the same way no matter where they are used. The chapter also presents an overview of the key concepts discussed in this book.