This chapter aims to outline the development of the performance framework for the 2017–2021 Mayoral Police and Crime Plan. This framework establishes what success looks like for the police and what they will be held accountable to deliver against by the mayor. The framework was a deliberate move away from blunt pan-London targets and was designed to be a more sophisticated and evidence-based approach incorporating a wider suite of metrics beyond traditional policing (i.e. high harms, local evidence-based decision-making, inequalities in public perceptions, and workforce representation). The chapter explores measuring a healthy police service and highlights the importance of effective implementation and assurance in the performance world. Finally, the chapter reflects on the missed opportunities and disjoins to date between evidence-based policing and that of performance and how the two brought together may be able to offer new opportunities in driving evidence-based working.