Objective: First, we give an overview of child psychiatric research in the Generation R Study, a population-based cohort from fetal life forward. Second, we examine within Generation R whether the functional polymorphism (5-HTTLPR) in the promoter of the serotonin transporter gene interacts with prenatal maternal chronic difficulties, prenatal maternal anxiety or postnatal maternal anxiety to influence child emotional development. Method: A total of 2,136 northern European children were genotyped for 5-HTTLPR and rs25531. Mothers reported chronic difficulties and anxiety symptoms at 20 weeks' pregnancy and when the child was 3 years old. Child emotion recognition was observed at 3 years, and child emotional problems were assessed with the CBCL/1 1/2-5 at 5 years. Results: There were consistent main effects of maternal difficulties and anxiety on child emotional problems, but no main effect of 5-HTTLPR. Moreover, children with the s allele were at increased risk for emotional problems if their mothers reported prenatal anxiety symptoms (beta = 2.02, p less than 0.001) or postnatal anxiety symptoms (beta = 1.64, p less than 0.001). Also, in children of mothers with prenatal anxiety symptoms, the s allele was associated with less accurate emotion-matching (beta = -0.11, p = 0.004). Conclusions: This population-based study shows that vulnerability due to 5-HTTLPR is not specific for certain adverse exposures or severe events, but suggests that the small effects of gene-environment interaction on emotional development become manifest early in life. (Contains 10 tables and 3 figures.)