Thyroid hormones are critical determinants of postnatal skeletal development. Thyroid hormone deficiency or excess in children results in severe abnormalities of linear growth and bone maturation. These clinical observations have been recapitulated in mutant mice and these models have facilitated studies of the mechanisms of thyroid hormone action in the developing skeleton. In this review, we consider in detail the direct and indirect effects of thyroid hormone on bone and the molecular mechanisms involved.