For more than a generation, education leaders have advocated for policies at federal, state, and district levels to support the spread of teacher leadership. And yet, teacher leadership has not taken hold in either a strategic or systemic way. Why? Perhaps, as Atul Gawande said of the medical industry, while good ideas abound, not all of them spread (2013). Ideas that do spread share two important qualities: They solve a clearly defined problem, and they benefit the general public as well as practitioners. The failure of teacher leadership to take hold has been a failure to define its purpose beyond the generic, albeit laudable, ideal of increased professionalism for teachers. Policy making is fundamentally about resource allocation. Policies favor interventions that can answer the key questions: Who benefits, how, and to what degree? Those who work in the teacher leadership space must ask: How do we make teacher leadership indispensable? How do we ensure that policy makers believe it is a priority and not the first item that gets slashed when budgets are tight? For teacher leadership to have staying power, it must prove itself to be genuinely influential--to matter more than other strategies for improving schools. Teach Plus has identified five measurable goals for teacher leadership: (1) Improve student outcomes; (2) Improve the access of high-need students to effective teachers; (3) Extend the careers of teachers looking for growth opportunities; (4) Expand the influence of effective teachers on their peers; and (5) Ensure a role for teachers as leaders in policy decisions affecting their practice.