Slavery in the pre-Civil War United States is a hard topic to teach, not only because it raises issues of racism and injustice, but also because students assume so much. Often, they think all northerners were abolitionists or "good guys" and southerners were "bad guys" who enslaved African Americans because they viewed them as inferior. England, if considered at all, is seen as a champion of the anti-slavery movement, having abolished slavery earlier in the nineteenth century. Textbooks form and reinforce these assumptions. In an effort to challenge these preconceived notions and spark students to think more deeply about slavery, two high school history teachers Cathy and Nancy were following the example of historians like Thomas Bender by placing U.S. history in a larger global context. As part of their participation in Words That Made America, a project funded by a Teaching American History grant, they developed a lesson to help students develop more complex understanding of U.S. slavery, and through the project's lesson study component, investigated how their students' understanding evolved. Not surprisingly, they found that no single lesson, no matter how well conceived or executed, replaces students' old understanding with a new one. What they did learn is that students' understanding evolves, and new knowledge and economic perspectives mix with prior knowledge and moral perspectives in a complex process of meaning making. "Pentimento," a term used in painting, provides a useful metaphor for understanding this process. Just as "pentimento" still shows an artist's earlier, differing conception and execution of a subject underneath the final layer of paint, students' new understanding from a lesson challenging prior assumptions still includes elements of those old ideas as they evolve. In this article, the authors talk about these evolutions in understanding and their implications for teaching about slavery in United States history. (Contains 31 notes.)