In this article students' understanding of trigonometric functions in the context of two college trigonometry courses is investigated. The first course was taught by a professor unaffiliated with the study in a lecture-based course, while the second was taught using an experimental instruction paradigm based on Gray and Tall's (1994) notion of procept and current process-object theories of learning. Via interviews and a paper-and-pencil test, I examined students' understanding of trigonometric functions for both classes. The results indicate that the students who were taught in the lecture-based course developed a very limited understanding of these functions. Students who received the experimental instruction developed a deep understanding of trigonometric functions. An activity to learn the process of computing sines and cosines using the unit circle is appended. (Contains 4 tables.)