The purpose of this study was to identify and explore the factors that supported or inhibited same-subject teacher-teams as they collaborated to develop common learning targets and common formative assessments, in year one of their high school's professional learning community. This qualitative study examined the experiences of teachers as team-members, primarily through interviews and a review of documents and artifacts. Eleven inhibiting and twelve supporting factors were identified and presented within a framework that conceptualized three primary sources for the identified factors: organization, team, and individual team members. Ways in which the factors reportedly supported or inhibited team productivity and affected professional satisfaction are presented. An analysis across teams suggesting relationships between factors and different levels of team success is also provided. Implications and recommendations are directed at educational leaders who seek to create the conditions necessary for successful, results-oriented, teacher-collaboration including suggestions for leveraging the supports and mitigating the inhibiting forces influencing teacher-teams. [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.]