Catalyst is designed to support and promote the change, professional learning and development of groups of teacher leaders, both within and across schools. It consists of two sets of evidence-informed resource cards and a clear and detailed facilitator guide. Catalyst is designed for schools in all contexts and across all student age groups. It is for use by and with middle leaders and other teacher leaders within and across schools, and the senior leaders who support teacher/middle leaders. Middle leaders have a formal role, with responsibilities for a subject, cross-curricular aspect of teaching and learning, social development of students, or for a stage or phase of schooling. Teacher leaders are informal leaders. Teaching is central for them, but they also choose to play a collaborative role in leading change and supporting the professional learning of colleagues in their school and other schools. Catalyst can also be used by: (1) senior leaders, for refreshment, reinvigoration and to further develop their capabilities; (2) senior leadership teams, to audit their own situation; (3) headteachers/principals interested in creating a supportive culture; (4) leaders supporting all colleagues across/in other schools; (5) colleagues involved in specific improvement programmes with an interest in change; and (6) facilitators of leadership development, especially teacher/middle leadership.