The present study used social learning theory and situated learning theory as a way to examine secondary social studies teacher participants' formal and informal professional learning. Existing literature is just beginning to attend to the potential of informal professional learning and to distinguish between formal and informal professional learning, so this exploratory study used observations of scheduled and spontaneous professional learning experiences, semi-structured interviews with 12 secondary social studies teachers, and relevant documents to consider the following research question: In what (...) ways do high school social studies teacher participants engage in formal and informal professional learning? Results indicate that while secondary social studies teachers engage in a variety of formal and informal professional learning experiences, their informal learning experiences are the most valuable and significant for their classroom practice. While informal professional learning is not a replacement for traditional formal professional learning, lessons learned here can inform ways to improve formal professional learning and to capitalize upon teachers’ existing informal professional learning experiences. This study provides insight into the ways in which social studies teachers support their growth outside of formal professional development. (shrink)