The focus of this work is aggressive behaviour in children of early and preschool age. Definitions and divisions of agressive behaviour are explained, as well as two primary theories of aggression occurrence: biological and environmental. The main factors of the biological theory are: genes, hormones, temper, age, sex and amygdala in the brain. The main factors of the environmental theory are: family impact, media impact and learning by model. This paper examines the main features of aggressive behaviour in children of preschool age and inside the kindergarten. The connection between aggressive behaviour and peer acceptance is researched. The role of the preschool educators, parents, professional associates and people close to the child is to redirect aggressive behaviours in other, socially acceptable behaviours. This paper also analyzes the forms of interventions inside the family home and inside the institution, including the cognitive-behavioral treatment. It is important not to suppress aggression, to show understanding and be patient towards children who display aggressive behaviours and to focus on the causes and solutions.