Catalyst is a professional learning tool intended to bring to life the findings of a collaborative research and development project carried out between researchers at the UCL Institute of Education and Challenge Partners, an informal national network of schools. The project investigated four questions:1.What is it about effective middle leadership within and across schools that changes teachers' practice? 2.What are the most powerful ways of sharing knowledge about excellent middle leadership practice within and across schools? 3.What are the most important factors when designing evidence-based tools to track changes in teachers' practice as a result of middle leaders' interventions? 4.What leadership conditions in schools help develop and embed cultures of shared outstanding practice? The collection consists of two sets of resource cards a clear and detailed facilitator guide - along with a bibliography and photocopiable resources for download - and has been designed for use by staff in schools in all contexts and across the entire student age range. Ultimately, the aim of the Catalyst process is to: help develop better understanding of and skills in teacher/middle leadership stimulate evidence-informed conversations about teacher/middle leadership which lead to deep and meaningful learning support skilled facilitation of these conversations encourage inquiry, improve problem-solving and enrich decision-making through collaboration enhance leadership within schools and across networks and school partnerships.