Insulin orchestrates physiological responses to ingested nutrients; however, although it elicits widely ramifying metabolic and trophic responses from diverse tissues, 'insulin resistance (IR)', a pandemic metabolic derangement commonly associated with obesity, is usually defined solely by blunting of insulin's hypoglycaemic effect. Recent study of monogenic forms of IR has established that biochemical subphenotypes of IR exist, clustering into those caused by primary disorders of adipose tissue and those caused by primary defects in proximal insulin signalling. IR is often first recognised by virtue of its associated disorders including type 2 diabetes, dyslipidaemia (DL), fatty liver and polycystic ovary syndrome (PCOS). Although these clinically observed associations are confirmed by cross-sectional and longitudinal population-based studies, causal relationships among these phenomena have been more difficult to establish. Single gene IR is important to recognise in order to optimise clinical management and also permits testing of causal relationships among components of the IR syndrome using the principle of Mendelian randomisation. Thus, where a precisely defined genetic defect is identified that directly produces one component of the syndrome, then phenomena that are causally linked to that component should be seen. Where this is not the case, then a simple causal link is refuted. This article summarises known forms of monogenic severe IR and considers the lessons to be learned about the pathogenic mechanisms both upstream from common IR and those downstream linking it to disorders such as DL, fatty liver, PCOS and cancer.