In order to improve animal welfare, a growing number of calves, heifers and dairy cows are reared in loose housing systems. However, (re)grouping unfamiliar animals may result in aggressive interactions and distress, especially for low ranked animals. Grouping of unfamiliar animals is found to increase aggression, social stress, locomotion behaviour and to have negative effects on feed intake and milk yield. Problems related to social integration are normally higher for the introduced animals than the resident animals. Previous social experience, number of animals that are mixed and the group composition are important factors that can influence social integration. Calves reared in groups are more social confident and show less fear than calves reared in single boxes or in isolation. Repeated grouping seem to accustom the animals to these procedures. Increased knowledge about social integration is necessary to develop effective management techniques that reduce the amount of problems occurring during social integration of cattle.