Technological advances, especially radioimmunoassays, have led to good descriptive information about the timing and pattern of development of the thyroid gland (TG) and circulating thyroid hormone (TH). Immunocytochemical studies in combination with more traditional techniques such as gland ablation and hormone replacement have revealed the time of appearance of each hormone in the axis, the relative amounts of hormone present in each gland at different developmental stages, and a general picture of the pattern of maturation of the hypothalamic-pituitary-thyroid axis. Studies of the pituitary control of the TG remain limited by the lack of adequately specific techniques for measuring avian thyroid-stimulating hormone. Our understanding of peripheral TH dynamics is progressing as a result of iodothyronine deiodinase assays but full understanding will require more elaborate studies that adequately characterize the enzyme system(s) in each case. Molecular techniques have made powerful strides in identifying the gene responsible for producing a protein that appears to be the triiodothyronine receptor. Receptor assays have revealed the pattern of changes in receptor-binding capacities during development but have not yet revealed how binding of hormone to the receptor triggers cellular activity. Molecular genetic techniques are being used to reveal the mechanisms whereby some examples of developmental events (e.g., malic enzyme synthesis) are induced by TH. Although it is not yet possible to assess the value of molecular studies in this area for developing practical applications, these techniques are revealing the fundamental biological roles of TH in growth and development.