This study explored first-semester calculus students' understanding of tangent lines as well as how students used tangent lines within the context of Newton's method. Task-based interviews were conducted with twelve first-semester calculus students who were asked to verbally describe a tangent line, sketch tangent lines for multiple curves, and use tangent lines within the context of Newton's method. We examined students' graphical illustrations and the language they used to describe tangent lines and identified six prominent categories that described students' concept images of tangent lines. Our data show that individual students often possessed multiple concept images of tangent lines, and often these multiple concept images were conflicting. Furthermore, students were usually willing to modify their concept images of tangent lines depending on the task presented to the students.