Previous research has revealed an association between early language difficulties and later internalizing problems, but less is known about the mechanisms behind this association; the timing, the direction of the associations and gender differences. In the current thesis, we have used different models to investigate this association longitudinally in a population-based sample. Results from a cross-lagged model showed that there were bidirectional relations between language difficulties and internalizing problems from 18 months to eight years, especially for girls and especially during the years of transition to school. Furthermore, we found support for a mediation model including two indirect pathways from language difficulties to internalizing problems, one through emotion regulation and one through social engagement. Finally, we showed that the longitudinal association between language difficulties and internalizing problems from five to eight years was best explained by a common underlying family factor shared by siblings. The results highlight the importance of increasing knowledge about language difficulties among mental health professionals, perhaps especially within family prevention in the primary health services. All papers were based on data from the Norwegian Mother and Child Cohort Study (MoBa).