The microdialysis technique has been widely used for the study of extracellular chemical events in various tissues. Until recently, however, little was known about the sampling characteristics of dialysis probes in vivo. The purpose of this review is to describe several theoretical models of in vivo microdialysis that consider both the diffusional resistances encountered in vivo and the effects of various active processes, such as metabolism, release and uptake, and transport across the blood brain barrier. The relevance of the models to practical in vivo microdialysis experiments is discussed. Several microdialysis techniques that quantitatively estimate extracellular analyte concentrations are also reviewed. These techniques are discussed in relation to the theoretical models, together with a discussion of the experimental factors that affect in vivo microdialysis experiments.