the measurement of metabolic fluxes in vivo in humans requires the infusion of labelled tracers and the analysis of tracer kinetics measured by mass spectrometry and analyzed by mathematical modeling. This talk will show how by using these techniques it is possible to measure in vivo hepatic glucose production (and the relative contribution of glycogenolysis and gluconeogenesis), peripheral (mainly muscle) glucose disposal, adipose tissue lipolysis, novo lipogenesis and the alterations often observed in subjects with metabolic diseases like type 2 diabetes and non alcoholic fatty liver disease.