Curcuminoids are safe natural yellow pigments used as food coloring agents and traditional drugs with a variety of biological functions such as antitumor, anti-inflammatory and antioxidant activities. Poor oral bioavailability and the low plasma concentration of curcuminoids limited their clinical use, and one of the major reasons is their rapid metabolism in vivo. The predominant metabolic pathways are reduction and conjugation, and some drug metabolizing enzymes such as alcohol dehydrogenase, UDP-glucuronosyltransferases (UGTs) or sulfotransferases (SULTs) involved in the metabolic reactions. Besides the major metabolic pathways, dehydroxylation, cyclization and methylation can also occur in vivo. In addition, more than thirty metabolites of curcuminoids have been identified in biological matrices including the plasma, urine and bile from rats or humans by LC-MS/MS analysis and other methods. Some metabolites such as tetrahydro-curcuminoids have been reported to be active, which may explain how and why curcuminoids with poor oral bioavailability display their effectiveness in vivo. The present review mainly summarizes curcuminoid metabolism and its contribution to the pharmacological effects.