Many children report feeling lonely sometimes or often. For most of those children, loneliness is a temporary experience, serving an adaptive function by motivating them to reconnect to others. Some children, however, feel lonely across long periods of time. Such prolonged loneliness has been found to be associated with academic difficulties and poorer psychological and physical health. This entry discusses the concept of loneliness in childhood, highlighting its emotional, cognitive, and genetic components. In addition, the entry considers how the family and peer environments contribute to children's feelings of loneliness. Interventions are proposed that focus on (1) helping those children who have moved from temporary to prolonged feelings of loneliness and (2) ensuring that children who report temporary loneliness do not move to prolonged loneliness. Such interventions include social skills training, providing social opportunities for youth, CBT and attention‐retraining programs, and addressing the stigmatization of loneliness among youth.