This qualitative study was prompted by initiatives that addressed the need for teachers to engage in professional development that enables them to be 21st century ready. Recommendations put forth by government and business have stressed that professional development foster connected teaching and create networked educators by emphasizing peer-to-peer collaboration and sharing. Despite this focus, little attention has been paid to the role that regular teachers play in becoming professional developers for their colleagues. My study investigated how four K-12 teachers, that I termed "knowledge broker teachers," created new pathways for informal, teacher professional development in their schools. Extending on the concept of "knowledge brokers" from business studies, knowledge broker teachers serve as an informal source of professional development, moving knowledge from those who have it to those who need it. This study's purpose was to examine examined how knowledge broker teachers built and shared their knowledge, and to identify their attributes. I applied a situated learning approach to frame this study, emphasizing the social nature of learning. Participants included four K-12 knowledge broker teachers and 12 of their teacher colleagues with whom they shared knowledge. Data collection included the use of interviews with participants and screen casts of the knowledge broker teachers' online activity. Data analysis employed open coding to generate categories, then themes. Three findings about knowledge broker teachers emerged: brokers, brokering, and brokerage. Brokers encompassed the context-dependent ways the four knowledge broker teachers shape-shifted and assumed different personas (e.g. knower-learner, comrade, cheerleader, shrinking violet) enabling them to be knowledge broker teachers. Brokering entailed the processes they used to build and share knowledge. These included processes of making connections through online and face to face opportunities, taking advantage of moments of kismet, and tailoring knowledge to match their colleagues' ability. Brokerage involved the actions that affected the quality of social relationships and the emergence of trust between the knowledge broker teachers and their colleagues. Brokerage actions presented by the knowledge broker teachers included giving and taking knowledge with colleagues, recognizing and honoring their colleagues' potential, and being expected to go above and beyond. My study recognized the existence of knowledge broker teachers and their effect on informal professional development. However, given the findings, formalizing their roles in schools may have a detrimental effect on their ability to build and share knowledge. Considering ways to leverage these findings may provide new ways for thinking about informal teacher professional development. [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.]