Quercetin is a typical anti-oxidative flavonoid ubiquitously distributed in vegetables. It is likely to act as a bioactive compound by exerting reactive oxygen species (ROS)-scavenging activity and/or binding to specific proteins such as oxidative enzymes and transcriptional factors in signal transduction pathways. Its absorption and metabolism (as well as its molecular targets) have been extensively explored from the viewpoint of its potential for disease prevention. It is known that glucuronide and/or sulfate conjugates with or without O-methylation exclusively circulate in the human bloodstream after intake of a quercetin-containing diet. We propose that glucuronide conjugates of quercetin function not only as detoxified metabolites but hydrophilic bioactive agents to various ROS-generating systems and precursors of hydrophobic aglycone. Quercetin aglycone is assumed to emerge in the target site by the action of β-glucuronidase activity under oxidative stress such as inflammation. The cardiovascular system and central nervous system seem to be the major targets of conjugated quercetin glucuronides circulating in the human bloodstream.