Group size may have a large impact on the behaviour of farm animals, especially in species where the natural group size is very different from the group size used on farms. In cattle, large group sizes are common in wild populations. For poultry and pigs, however, the group size found in the wild is generally much smaller than the group size found under farming conditions (Collias et al., 1966; Gonyou, 2001). Large group sizes may lead to damaging behaviour, aggression and increased fear and stress levels. The aim of this review is to study the impact of group size on these parameters and to identify housing and management options that can help to reduce problems caused by increased group size. Increasing group size was found to increase the risk of damaging behaviour, such as feather pecking in laying hens and vulva biting in sows. Aggression does not appear to be a problem in large groups, because dominance relationships in these groups are not based on individual recognition, but based on other signals such as body size, avoiding costly fights. There is evidence for increased fear and stress levels in large groups compared with small groups, but fearfulness is also strongly affected by type of housing. To minimize problems in large groups it seems helpful to offer separate functional areas and to provide cover, reducing disturbance between animals. To minimize the risk of damaging behaviour, such as feather pecking in laying hens and tail biting in pigs, stimulating foraging, exploration and manipulation behaviour by providing sufficient substrate (straw, wood shavings and sand) offers perspective. Rearing the animals in a system that allows the development of all these behaviours is very important. Other solutions can be found in optimizing the diet and offering extra foraging opportunities. Furthermore, genetic selection against damaging behaviour seems promising. In conclusion, group size mainly has an effect on damaging behaviour and fear and stress in pigs and poultry. The effect on aggressive behaviour is limited. To reduce damaging behaviour, fear and stress, it is important to provide a complex environment with ample behavioural opportunities and separate functional areas.