The majority of UDP-glucuronosyltransferases (UGT), like other drug-metabolizing enzymes, display broad and often overlapping substrate specificities, complicating evaluation of the function of individual UGT isoforms within human tissues. Despite this, there have been recent advances in identifying UGT-selective probes--UGT substrates that are primarily glucuronidated by a single isoform. Such probes can be used to (1) facilitate identification of UGT isoforms mediating a particular glucuronidation activity in human liver through activity correlation analysis; (2) evaluate the role of particular UGTs in drug-drug interactions through either enzyme induction or inhibition; and (3) elucidate the functional significance of genetic polymorphisms associated with the gene encoding the UGT of interest. UGT-selective probes currently being used in our laboratory for the evaluation of glucuronidation activities in human liver tissues include estradiol (3OH-glucuronidation; UGT1A1), trifluoperazine (UGT1A4) serotonin (UGT1A6), propofol (UGT1A9), 3'-azidothymidine (UGT2B7), and S-oxazepam (UGT2B15). In vitro incubation protocols and the HPLC analysis methods used to determine each of these activities are described in detail. Future work is needed to elucidate more highly selective probes than those in current usage, as well as probes for the extrahepatic UGT isoforms.