Many early language support programmes have been implemented in ECEC settings over the last years in Germany. Most of them are targeted programmes which have not shown to have longer term beneficial effects. The concept of early language education embedded into daily preschool routines is a child-oriented approach for all children and all age groups. The national programme "core daycare centres language & integration" aimed at promoting early language education embedded into daily routines at 4000 participating centres. In this paper, the longitudinal evaluation study of the programme is described, and the specific methodological challenges and potentials of the evaluation are discussed. The lack of instruments in ECEC research, the nature of pedagogy in ECEC, the autonomy given to the settings and professionals, the lack of control and the interpretation of effect sizes are highlighted as the main challenges. At the same time, the urgent need of such evaluations and the benefits for policy-makers, research, practitioners and the public are explored. (author abstract)