Early Childhood Education programs aim at preventing educational delays associated with socio-ethnic disadvantage in the home environment of young children. Advocates claim that such programs can be effective, provided they are of high quality. Despite the investment of enormous budgets, the educational gap between socio-economically deprived families and their wealthier counterparts is still widening. The question therefore is justifiied whether these claims are warrantjustified. This article focuses on the internal and external validity of the most cited preschool program, the High/Scope Perry Preschool Project, which was carried out between 1962 and 1967 in one school in Ypsilant, MI. Are the program's effects as reported by, e.g. Lawrence Schweinhart and James Heckman, reliable and valid? And is it really possible to generalize the findings of this so-called model program to other programs, target groups, ssettings and conditions, and target groups, as is being claimed?