The purpose of this qualitative phenomenological study was to explore and describe the practices of teacher leaders in a PLC content team along with the supports they received and needed as well as the challenges they encountered during implementation of the Common Core State Standards for high school mathematics in a Southern California school district. Because research shows most change initiatives require active leadership and yet are difficult to sustain there existed a need to study and describe the experiences of the teacher leaders during this curricular change to the Common Core State Standards. The experiences described by the seven teacher leaders participating in this study strengthen the practices of using collaborative content groups lead by teacher leaders. Common themes of teacher leader practices included having updated knowledge and skills about the reform through multiple trainings, honest and trusting relationships with their peers, sharing experiences and ideas, along with a shared vision and purpose, timelines and agendas. The supports received by the teacher leaders included the support of leadership through a TOSA or coach, the structure and time of the collaborative PLC content teams and on-going professional development. The challenges experienced by the teacher leaders were the need for more time and materials. The conclusions confirm that teacher leaders are the experts in their subject who practice building relationships through sharing of experiences and focusing on the goals for student improvement. Nine conclusions from this study confirm that the important practices of teacher leaders are knowledge of the change in curriculum to provide a common vision and purpose for student learning, using norms, agendas and timelines to stay focused on the goals, building trusting relationships and an environment of trying new things and sharing experiences. The leadership supports required include the PLC team time and structure including stronger administrative direction at the beginning and more choice as the work continues, the support of a TOSA or coach who advocates for the teachers and teacher leaders through communication, direction and resources and the additional support through on-going and continual professional development. The needs and challenges of the teacher leaders include more time to development, assess, adjust and improve the curriculum and the need for materials for both the teachers and students