Microdialysis has been used for 25 years to study brain function in vivo. Recently, it has been developed for investigations on peripheral tissues. A microdialysis catheter is an artificial blood vessel system which can be placed in the extracellular space of various tissues such as adipose tissue and skeletal muscle in order to examine these tissues in situ. Molecules are collected from the tissue by the device and their true interstitial concentration can be estimated. Metabolically-active molecules can be delivered to the interstitial space through the microdialysis probe and their action on the tissue can be investigated locally without producing generalized effects. It is also possible to study local tissue blood flow with microdialysis by adding a flow marker (usually ethanol) to the microdialysis solvent. The microdialysis technique is particularly useful for studies of small and water-soluble molecules. A number of important observations on the in vivo regulation of lipolysis, carbohydrate metabolism and blood flow in human skeletal muscle and adipose tissue have been made recently using microdialysis.