We implemented a sequential mixed methods design using parallel samples to answer our general research question: What are high school students' definitions of intelligence and implicit beliefs about the malleability of intelligence? We surveyed 9th and 11th grade students who responded to questions about their intelligence beliefs on open- and closed-ended items (Stage 1, n = 73; Stage 2, n = 150). Themes developed in the qualitative analysis were replicated closely in the quantitative analysis. We documented that high school students' definitions of intelligence can be explained by 3 broad factors in which intelligent students are portrayed as exhibiting thoughtfulness (i.e., making good decisions and caring about the well-being of others), putting forth effort in their academic endeavors, and having high abilities. Furthermore, many high school students believed that intelligence was malleable. (Contains 11 tables.)