Soon after the discovery of insulin, the importance of insulin sensitivity assessment was recognised. Recently, the role of insulin resistance in cardiovascular morbidity and mortality has been widely accepted and many methods for in vivo assessment of insulin resistance have been developed. They may be classified as 'closed-loop' techniques (in which insulin and glucose concentrations are allowed to interact freely), 'open-loop' techniques (in which insulin and/or glucose levels are fixed) and 'model methods' (which use a mathematical model to analyse the interactions between insulin secretion patterns and glucose disposal). Although there is no ideal method available to date, open-loop techniques avoid many of the difficulties involved in the interpretation of closed-loop or model methods, and are preferred by most investigators. The glucose clamp technique is recognised as the 'gold standard' for assessment of insulin sensitivity; however, the insulin suppression test is adequate in most circumstances and is much simpler to perform.