The studies presented in this thesis have been conducted as part of the Netherlands Consortium for Research in Child Care (Nederlands Consortium Kinderopvang Onderzoek, NCKO) research program, which has received funding from the Dutch Ministry of Social Affairs and Employment since 2002. The general purpose of the NCKO project was to assess and to improve child care quality for 0- to 4-year-olds in The Netherlands. The main aim of this thesis has been the development and evaluation of an intervention program to improve the quality of child care. The first study describes the validation of the Caregiver Interaction Profile (CIP) scales. Next, this thesis reports on the application of the CIP scales in combination with the widely used Infant/Toddler Environment Rating Scale-Revised (ITERS-R) and Early Childhood Environment Rating Scale-Revised (ECERS-R) in a nationally representative sample of Dutch child care centers. Outcomes of this second study were the starting point for quality improvement. To this purpose, we developed a broad intervention program that aimed to improve both the quality of caregiver-child interactions as assessed with the CIP scales (described in study 3) and the more global quality of the child care environment as measured with the ITERS-R/ECERSR (described in study 4) in the same care group.