Catch-up growth is a regular phenomenon seen in young children after a period of growth retardation when the cause of the growth deficit is removed. It consists of an immediate growth acceleration with an abnormally high velocity, followed by a progressive deceleration till the original or normal growth channel is reached. Examples of individual patients and groups of patients with endocrine disorders, metabolic disorders, malnutrition and prenatal growth retardation have been given. Clinical experience shows that complete or near complete catch-up growth is possible in infants and young children, but not in children near or in puberty. The factors responsible for catch-up growth and the regulating mechanisms are unknown. Serum growth factors, receptor changes or genetically programmed cells are more likely to be involved than the classical growth promoting hormones. Superficially catch-up growth is a growth spurt similar to the pubertal growth spurt. However, the shape of the velocity curve, the maturity stage at which it occurs and the mechanisms involved are entirely different. The opposite of catch-up growth which compensates for a previous growth deficiency is the lagging-down growth which compensates for a previous growth excess. Both forms of growth compensation are examples of the general biological phenomenon of canalization of growth and maturity.