Social interaction plays a crucial role in human evolution, development, and everyday social life, so there are numerous areas of social psychology to explore, such as group dynamics, bystander intervention, crowd behaviour, impression formation, the psychology of deceit, and of social behaviour. ‘How do we influence each other? Social psychology’ considers the field of social neuroscience as well as studies exploring indirect and direct influences. The impact on human behaviour of the presence of others is called social facilitation. The social behaviour effects of leadership, conformity, obedience, and prejudice are considered. Future challenges are to find out more about facts that predict, control, or modify different types of behaviour and more about the cognitive neuroscience of social interaction.