It is a paradox that thyroidology often initiates new biological concepts but lags behind in the characterization of the molecular mechanisms involved, the latter progressing more rapidly in other systems. The question of the TSH receptor is an illustration of this. This paper reviews the main aspects of the biochemistry and functions of the TSH receptor. It is stressed that the classical biochemical approach has failed in the elucidation of the structure of the receptor which should benefit from molecular biology methodologies. Experimental difficulties in the study of the TSH binding function of the receptor are also described. Functional pathways activated by the receptor are discussed, especially within the frame of current researches on the thyroid growth effect of TSH. Another important feature concerning the TSH receptor is its implication in the physiopathology of Graves' disease. Auto-anti-TSH receptor antibodies are the hallmark the disease and are responsible for the hyperfunction of the gland. Characterization of the antigenic epitopes of the receptor and biological effects of the various anti-TSH receptor antibodies, both natural polyclonal and experimental monoclonal, is also an area of active research. It is hoped that understanding of Graves' disease will benefit from improvement in our knowledge of the TSH receptor structure and functions.