This study examined the extent to which an evidence-based preschool curriculum (Head Start REDI) was sustained by 20 teachers during the year following a randomized controlled efficacy trial, when teachers were no longer required by the research project to implement the curriculum. Two quantitative measures of sustainability (teacher ratings, REDI coach ratings) and a qualitative measure (teacher interview) were collected and compared. Sustainability varied by the specific curriculum component, with higher rates of sustainability for the social-emotional component (Preschool PATHS) than for the language and literacy components. Estimates of sustainability were affected by the method of measurement, with REDI coach ratings and qualitative teacher interviews more closely aligned than teacher ratings. Responses from qualitative interviews identified the main factors that teachers thought affected sustainability. Teacher responses suggest that efforts to promote sustainability are best targeted at reducing barriers, such as competing demands, rather than simply highlighting the benefits of the new curriculum. (Contains 3 tables.)