After 20 years in the wilderness, 3,5,3′‐triiodo‐l‐thyronine (T3) has come back home, for it is exactly 20 years since Gross&Pitt‐Rivers (1952) first drew attention to its presence in human serum. A few years later Pitt‐Riverset al. (1955) demonstrated its formation from thyroxine (T4) given to patients with athyreotic myxoedema, and in 1957 Maclagenet al. reported thyrotoxicosis with elevated plasma concentration of T3 and normal concentration of T4‐the first identifiable instance of what is now referred to as ‘T3 toxicosis’. Despite these early suggestions of significant physiological and pathological contributions, T3 was regarded as a minor and unimportant iodinated compound until the development of an improved biochemical assay (Sterlinget al., 1969) paved the way for wider recognition of its role in health and disease and stimulated the development of a sensitive and dependable radioimmunoassay. In the past few years information about the action, metabolism and clinical significance of T3 has rapidly accrued; this review attempts to put together some of this new and exciting information. Because methods for the assay of T3 in biological fluids have only recently been introduced, the validity of the various techniques will be