Adequate control of immunosuppression is critical in preventing graft failure after solid organ transplantation (SOT) and in avoiding life-threatening viral and malignant complications. Prolonging patient and graft survival and delaying re-transplantation as children reach adulthood is critical to optimal use of a scarce resource. This requires tailoring post-transplant management to the unique needs of the child. Immunosuppression management is challenging in infants, children and youth. The interval from birth to young adulthood sees profound changes in physiological processes, body size and immune maturation; infancy and adolescence are the periods of most rapid and dramatic change. Three pivotal factors affect immunosuppression control in the child: 1) age-dependent variation in drug metabolism; 2) developmental changes in immune function with increased childhood susceptibility to infections, including those caused by viruses; and 3) behavioural changes in adolescence and young adulthood linked with poor treatment adherence.