Links between individual differences in 4-year-olds' social understanding, language abilities, temperament, behavioral adjustment and family background and the quality of their interaction with a close friend were investigated. 64 pairs of friends were filmed playing together on two occasions, and each child tested on a battery of assessments of theory of mind, emotion understanding and language. Teachers and mothers reported on children's adjustment and temperament respectively. There were marked differences in children's interactions with their friends; the sociocognitive abilities and behavioral characteristics of both child and friend contributed significantly to cooperative shared pretend, to low frequency of conflict and to successful communication between friends, behavioral adjustment and family background also contributed independently, to friendship quality. The similarity, between friends in behavioral adjustment and sociolinguistic skills was notable.