This study profiles and compares international social contexts for teacher workplace informal learning from the teachers' perspectives. Set in elementary schools in the U.S. and Lithuania, the study illustrates how teachers make sense of and engage in professional learning within their historical, political and administrative contexts. A socio-cultural framework brings into view different opportunities for teacher informal learning. These evolve from comparing teacher interviews about their learning to ethnographic case studies of the school's cultures that include schools' missions, building structures, classroom environments, organizational arrangements, traditions, and professional relationships. The study illustrates key interrelated cultural qualities of schools that support and hinder informal learning; argues for the importance of acknowledging teacher informal learning as a method of career-long professional development, and suggests further research about how to build and sustain the school infrastructure necessary to maintain such development.