Effective active learning supports student engagement in generative learning behaviors, often through collaborative group work. Although collaborative learning has many benefits, it can be difficult for individual instructors to prompt knowledge building discourse in large enrollment courses. Peer Learning Assistants (PLAs) can provide the instructional assistance needed to facilitate student engagement in active learning. This research explored how PLAs supported learning while students worked collaboratively in groups in an introductory biology course. I used a peer-led interactive discourse framework to investigate how PLAs facilitated student engagement in knowledge building discourse. This framework was informed by the ICAP modes of engagement (Interactive, Constructive, Active, Passive), which posits that students' cognitive engagement in a learning task is exhibited through their overt behaviors. Using discourse analysis to analyze audio recordings of PLA-student interactions as student worked collaboratively in groups, I described PLA practices and student actions that were pertinent to student learning. Then, I applied the ICAP framework to characterize each PLA-student interaction as Interactive, Constructive, Active, or Passive based on the groups' actions. All interactions were also given an Interaction Quality Score to quantify the quality of student participation in the interaction. I compared the ICAP characterizations and Interaction Quality Scores of PLA-student interactions between groups to investigate how student participation and group dynamics mediated student performance. I describe four main conclusions: 1. Students were less engaged in generative learning actions when PLAs used informing and mediating practices; 2. Students engaged in generative learning actions when interacting with PLAs that utilized probing and guiding practices; 3. Group dynamics influenced the nature of PLA-student interactions, such that not all students participated equally; and, 4. The nature of student engagement in PLA-student interactions may mediate student performance. I found that student participation in Interactive interactions was the most supportive of student performance, and students who engaged with the PLA during these interactions performed better than their less engaged peers. This research offers implications for PLA pedagogy training and instructor decision-making regarding effective implementation of active learning to support student engagement in knowledge building discourse. [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.]