In the four studies comprising this dissertation, we aim to shed more light on the role of implementation quality of FLPs. In the first study, we reviewed the literature on implementation quality of FLPs, to gain insight into how implementation has been assessed in studies so far, what previous studies reveal on (systematic differences in) the implementation of FLPs, and the extent to which implementation quality relates to intervention effects. In the second study, we measured and analyzed implementation quality and effects of a specific Dutch FLP called ‘Early Education at Home’. In light of the hypothesis posed in the literature regarding implementation quality of FLPs in low-SES families, and our hypotheses regarding implementation quality of FLPs in ethnic-minority and bilingual families, we additionally analyzed relations among these parent characteristics and program implementation. In the third and fourth study we took a more detailed look at parent-child interactions during literacy-related activities included in FLPs such as EEH: shared reading and socalled prompting board activities. Observations of these activities can provide insight into how target group parents respond to different FLP activities and, more specifically, if certain activities are more suited to some groups than to others.