For three years from 2008 every school in England had a designated school improvement partner (SIP), portrayed as a critical friend, whose role was to support and challenge the headteacher. A mixed-methods study involving a national survey and face-to-face interviews evaluated the enactment of the national policy from the perspective of the direct recipients--the headteachers/school principals. Headteachers' perceptions of their school improvement partners, and their experiences of the support and challenge provided by SIPs, varied. Much seemed to depend on individual SIP's expertise and conduct. The SIPs' prescribed agenda was seen as too focused on data rather than discussions about learning and teaching, and requirements for SIPs to report to the local authority and governors were in tension with trustful relationships with headteachers. The SIP programme could be interpreted as a commitment to the entitlement of headteachers to support and challenge, or as a mechanism for surveillance and discipline. Lessons are drawn for the "national" and "local leaders of education" who have replaced SIPs, and for anyone internationally concerned with support and challenge for school principals.