"A Framework for K-12 Science Education" identifies eight science and engineering practices for K-12 classrooms. These practices, along with core ideas and crosscutting concepts, define the nation's learning goals for science. An important advance from earlier standards (AAAS 1993, NRC 1996), these practices are clearly identified "not" as separate learning goals that define what students should know "about" the process of science. Instead, the scientific practices identify the reasoning behind, discourse about, and application of the core ideas in science. The practices outlined in the framework are: (1) Asking questions and defining problems; (2) Developing and using models; (3) Planning and carrying out investigations; (4) Analyzing and interpreting data; (5) Using mathematics and computational thinking; (6) Constructing explanations and designing solutions; (7) Engaging in argument from evidence; and (8) Obtaining, evaluating, and communicating information. In this article, the authors examine the sixth and seventh practices concerning explanation and argumentation, respectively. The two practices depend on each other: For students to practice explanation construction, they must also engage in argumentation. The authors first define argumentation and explanation individually and then explore their relationship in classroom examples. (Contains 1 figure.)