The Next Generation Science Standards (NGSS)--now in development--will be based on "A Framework for K-12 Science Education" released by the National Research Council last summer. The "Framework" identifies seven scientific and engineering practices that should be used in science classrooms. These practices reflect the multiple ways in which scientists explore and understand the world and the multiple ways in which engineers solve problems. These practices include: (1) Asking questions (for science) and defining problems (for engineering); (2) Developing and using models; (3) Planning and carrying out investigations; (4) Analyzing and interpreting data; (5) Using mathematics, information and computer technology, and computational thinking; (6) Constructing explanations (for science) and designing solutions (for engineering); (7) Engaging in argument from evidence; and (8) Obtaining, evaluating, and communicating information. In this article, the authors look in-depth at scientific practice #2--developing, evaluating, and revising scientific models to explain and predict phenomena--and what it means for classroom teaching. (Contains 4 figures.)