Previous research has reported a procedural, rather than conceptual, approach in college level Calculus courses. In particular, previous studies have shown that theoretical thinking is not a necessary condition for success. This can be gleaned from the exercises on assignments and assessments which constitute all, or most of, students’ course grades. This approach has been linked with institutional constraints that are often imposed on these courses. Our belief is that theoretical thinking is necessary for learning Calculus, and that students should be provided with opportunities to engage in this type of thinking. In this thesis, we provide empirical evidence that students can be engaged in theoretical thinking in a college level Calculus course, despite the existing institutional constraints. Students enrolled in a Calculus course were presented with optional tasks intended to engage them in theoretical thinking. We analyze collected data from the perspective of Sierpinska, Nnadozie, and Oktac’s (2002) model of theoretical thinking; all students attending class engaged in these optional tasks and our analysis shows that on average, more than half of them engaged in theoretical thinking. We place our study in the context of previous research in the teaching and learning of university introductory (and remedial) level mathematics and of the role that Calculus courses play in the mathematics education of undergraduate students.