The design and development of a new method for high school pjysics instruction is described. Students are actively engaged in understanding the physical world by constructing and using scientific models to describe, explain, predict, and to control physical phenomena. Course content is organized around a small set of basic models. Instruction is organized into modeling cycles which move students systematically through all phases of model development, evaluation, and application in concrete situations-thus developing skill and insight in the procedural aspects of scientific knowledge. Objective evidence shows that the modeling method can produce much larger gains in student understanding than alternative methods of instruction. This reveals limitations of the popular "cooperative inquiry" and "learning cyle" methods. It is concluded that the effectiveness of physics instruction depends heavily on the pedagogical expertise of the teacher. The problem of cultivating such expertise among high school teachers is discussed at length, with specific recommendations for action within the physics community.