The present study used Nardi and O’Day’s (1999) information ecology theory, along with activity theory (Wertsch, 1998), social learning theory (Wenger, 1998), and situated learning theory (Lave, 2009; Lave & Wenger, 1991) to examine the informal professional learning of a high school social studies department. Existing literature is just beginning to attend to the potential of informal professional learning, so this exploratory study used a single-case study of a high school social studies department made up of 12 teachers. Data included observations of scheduled and spontaneous collaborative learning activities, department meetings, and in-service meetings; semi-structured interviews; and relevant documents to consider how high school social studies teacher participants navigate their own informal professional learning. Supporting research questions included: (1) How do high school social studies teacher participants choose what to do to individually and collectively meet their professional learning needs? (2) What actions do participants take to meet their professional learning needs individually and collectively? (3) How do participants evaluate their professional learning growth individually and collectively? (4) How do participants interact with one another and with the environment as they navigate their own professional learning? Results indicated that participants valued their informal professional learning experiences, engaged in reflection throughout their informal professional learning, were influenced by departmental leadership, and experienced successes and failures in their informal professional learning goals. The department’s informal professional learning was important but also imperfect. Further consideration of teachers’ informal professional learning may offer new ways to support teacher growth.