Microdialysis is a minimally invasive tool that allows us to gain insight into metabolism at the tissue level. In human investigations, it can be safely performed in the brain (neurosurgical patients), skeletal muscle and adipose tissue. Basically, the technique allows interstitial concentrations of small solutes to be evaluated. Several limitations of the method and possible ways to circumvent them are indicated. Recent technical developments are reviewed. At present, this method is rarely used in metabolic monitoring of critically ill patients, but its potential applications are highlighted.