Background: The success of the learning assistant (LA) model has largely been attributed to LA facilitation of active learning tasks. A deeper understanding of how LAs facilitate these tasks would inform LA training and support successful adoption of the LA model. Our investigation of LA actions during their interaction with students in the classroom contributes to that understanding. We present and discuss the development of the action taxonomy for learning assistants (ATLAs), as well as illustrate its applicability by presenting some analyses that were conducted on sample data. Results: The LAs carried out several different actions that we categorized broadly as LA-Directed Facilitation, LA-Guided Facilitation, Advice, Feedback, Course-Related Talk, and Non-Course-Related Talk. LA-Directed Facilitation and LA-Guided Facilitation were the most common types of actions observed. We found that LA actions varied by course. Conclusions: ATLAs is a tool that can be used to examine LA actions. In our sample data set, LAs undertook many different actions during interactions with students which indicates that LAs play several different roles in the classroom. These findings have practical implications not only for faculty seeking to implement a peer instruction model such as the LA model, but also for instructors wanting to utilize LAs in their courses more effectively. Understanding what the LAs are doing during interactions with students can provide us insight into the different roles that LAs undertake. Knowledge of these roles will guide effective training, feedback, and direction of LAs, particularly during the pedagogy course.