Catch-up growth is characterized by height velocity above the limits of normal for age for at least 1 year after a transient period of growth inhibition; it can be complete or incomplete. Although catch-up growth can be expressed in terms of height velocity, the change in height standard deviation score is more appropriate. Catch-up growth is difficult to distinguish from the pubertal growth spurt. The increased growth rate following intrauterine growth retardation is usually called catch-up growth, although it does not meet all the criteria. It is not possible to know whether catch-up growth is complete for an individual child, but if final height is within the target range, it can be considered that catch-up growth has probably been complete. In groups of patients, complete catch-up growth is expected to result in a mean final height close to the mean target height. Increased growth velocity due to growth hormone (GH) therapy is accurately called catch-up growth in children with GH deficiency but should be called growth enhancement in children with other disorders. Two hypotheses have been proposed to explain the mechanism of catch-up growth: the neuroendocrine hypothesis, for which no persuasive experimental data have been produced, and the growth plate hypothesis, which cannot explain the increased growth rate observed in human catch-up growth.