Constipation is a common problem in children, with childhood prevalence estimated at between 1 and 30%. It accounts for a significant percentage of referrals to paediatricians and paediatric gastroenterologists. It commonly runs in families, suggesting either an underlying genetic predisposition or common environmental factors, such as dietary exposure. The peak age for presentation of constipation is shortly after toilet training, when passage of hard stools can cause pain on defecation, which then triggers holding-on behaviour in the child. At the time of the next call to stool the toddler may try to prevent defecation by contraction of the pelvic floor muscles and anal sphincter. Unless the holding-on behaviour is quickly corrected by interventions to soften faeces and prevent further pain, the constipation can very rapidly become severe and chronic. Until recently, this mechanism was thought to be the only significant primary cause of constipation in childhood. In this review, we will summarise recent evidence to suggest that severe chronic constipation in children may also be due to slowed colonic transit.