Scientifically literate citizens need to understand how scientists evaluate competing explanations. Likewise, students must learn to critically evaluate the quality of scientific knowledge and weigh alternative explanations. Regrettably, high school graduates often are not critically evaluative about scientific topics. To help remedy that, this article presents an instruction scaffold--called a model-evidence link (MEL) diagram--designed to promote students' ability to critically evaluate scientific arguments and deepen their understanding of fundamental science concepts. First, however, a discussion of the role of critical evaluation in promoting argumentation in the science classroom is provided.