Despite major changes to teacher evaluation since 2009, scant research examines how principals enact these policies. Drawing on qualitative interviews with 44 principals in 13 Connecticut districts, we use structure-agency theory to characterize how principals improvised when implementing the state evaluation model. We find that principals' use of discretion varied across the system's components, took different forms, and appeared aimed at varied outcomes. Particular forms of discretion supported the system's goals, while others likely undermined them. Principals tended to use their discretion to further the system's development aims as opposed to its accountability goals. Our findings have implications for the enactment of teacher evaluation policy, the roles of district administrators, and principals' work as instructional leaders in an accountability context.