This article reports on the role of the external adviser to school governing bodies in relation to headteacher performance management (HTPM) in England. School leaders are crucially important for the effective functioning of schools and there is an ever-growing body of inspection and research evidence which shows this to be the case – leadership matters, in particular principal or headteacher leadership. Oversight of the performance of school leaders is therefore crucial in the English educational system due to current initiatives promoting school autonomy coupled with tight, centralised accountability. Researchers in other countries have noted the lack of systematic understanding of the performance management of senior school leaders. The same applies to England; however, current changes to the educational system, including increased school autonomy and the centrality of school improvement, linked to inspection, make the stakes much higher than elsewhere. Understanding HTPM, especially the role of the external adviser or school improvement partner, is crucial given the evolving structure of the educational system and the shifting relationship between the state and schools. The role of the external adviser is especially pronounced in helping the governing body set objectives for its headteacher and make decisions about remuneration and salary increases. The article considers the degree to which outside assistance enables governing bodies to complete their key role in relation to managing the performance of headteachers and it outlines the main constraints under which external assistance and support operates. The article considers the role of external advisers as critical friends and concludes that good governing is at the heart of effective HTPM; the two are complementary. Governing bodies and boards of trustees will play a key role in the future, a role which can be made easier with the help of effective external advisers.