The syndromes of insulin resistance are a group of clinically diverse disorders, and our understanding of their molecular pathogenesis has advanced in parallel with our understanding of the structure of the insulin receptor and the mechanism of insulin action. The most straightforward progress has related to defining the role of both anti-receptor antibodies and mutations in the insulin receptor gene in causing these disorders. Despite this progress, the cause of severe target cell resistance in patients without defects in the receptor locus remains unknown, and we are limited in our ability to relate specific molecular defects in insulin signalling to in vivo phenotypes, such as those relating to growth and development and function of adipose tissue and muscle. Answers to these questions may ultimately be explained by the existence of multiple species of insulin receptors expressed in different tissues, brought about by alternative splicing and receptor hybrids, and by divergent pathways of insulin signalling with different consequences for specific tissues. The possibility that the insulin receptor and GLUT4 may be candidate genes for inherited insulin resistance in NIDDM has been addressed with the aid of genetic screening techniques such as SSCP. Currently, the loci have not been implicated in studies in most patients. Transgenic methodologies will be powerful tools for pursuit of unanswered questions in the field of insulin resistance in coming years.