This paper reports on the findings in the first phase of a design-based research project as part of a large-scale intervention study in Dutch kindergartens. The project aims at enhancing differentiated instruction and evaluating its effects on children's development, in particular high-ability children. This study investigates relevant intervention fidelity factors based on [Fullan, M. (2007). "The New Meaning of Educational Change." New York: Teachers College Press]. A one-year intervention in 18 K-6 schools was conducted to implement the screening of children's entry characteristics, differentiation of (preparatory) mathematics and language curricula, and a policy for the differentiation and teaching high-ability children. The intervention fidelity and implementation process were scored for each school using data from observations, field notes and log books. Self-report questionnaires measured participants' perceptions of the intervention (n = 35 teachers, 18 principals). Quantitative results showed that intervention fidelity differed between schools. Qualitative analyses of perceptions and cross-case analyses of three kindergartens showed that a strong need, pressure from parents, an involved principal, and teacher time and motivation contributed to successful implementation. Implementation barriers were the innovation's complexity, teacher beliefs, an absent principal and low teacher motivation (which was partly due to communication problems). Implications for interventions in general and differentiated instruction for high-ability children in particular are discussed.