The research project “Gezin en Persoonlijkheid” ("Personality and Family Relationships") was a temporary joint venture of four departments of the Faculties of Social Sciences of the University of Utrecht and the University of Nijmegen. Its principal goal was to study the transactional development of family relationships and personality in families with adolescent children. To this end a longitudinal data collection was designed and executed. In the project we studied 288 families in three measurement waves with an interval of one year, and using a cohort sequential design. In a round robin design family members gave self- and other ratings about their family relationships and relational support, personality, problem behavior, well being, identity and other. Additionally, similar information was collected in best friends of family members in the second measurement wave.