A growing body of literature focusing on undergraduate biology education calls for students to engage in the ways of thinking, feeling, and doing that reflect those in which biologists engage. Such reform efforts suggest that learning is best fostered when students engage in practices, concepts, and scientific reasoning as they endeavor to explain phenomena. However, it is recognized that if this experience is to reflect authentic science, such work requires that students learn to wrestle with uncertainty. The embrace of uncertainty as a component of rigorous science learning experiences is relatively new and unexplored in undergraduate settings. Each of the three research projects reported here are situated in a nonmajors biology course--and explain what makes this a useful context. In this dissertation, I offer a curricular analysis of the nature of uncertainty structured in a non-majors' biology laboratory, a description of instructional moves teaching assistants (TAs) use as their students encounter uncertainty, and a case study of three TAs coming to understand scientific uncertainty and epistemic affect in an internship course. In the first study, a curricular analysis revealed six types of uncertainty that were structured within the non-majors curricula, including analytical uncertainty, conceptual uncertainty, instrumental uncertainty, measurement uncertainty, procedural uncertainty, and signal uncertainty. In the second study, I described the instructional moves TAs used as their student's encountered uncertainty and how uncertainty is positioned for the students through their moves. TAs used instructional moves to position the uncertainty for students in specific ways: to raise the uncertainty, maintain the uncertainty, reduce the uncertainty and avoid the uncertainty. TAs' usage of these moves has implications for the types of uncertainty students encountered during tasks. In the final study, I examined how three TAs came to understand the interplay of scientific uncertainty and epistemic affect and the resources they drew upon as they made sense of this interplay. The three TAs developed personal definitions of uncertainty that largely cantered around the procedural aspects of science. The TAs drew upon five different resources as they made sense of uncertainty: their experiences as a TA, experiences as a student, experiences as a researcher, personal experiences, and experiences as part of the internship course. The TA's understanding of uncertainty through this experience has implications for both TA and teacher professional development. I intend for this dissertation study to support understanding undergraduate non-majors' biology laboratory courses and the opportunities for students to engage in authentic science to develop biological proficiency. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]