Nitrate contamination of groundwater often results from human activities. Stable isotope ratios of N (15N/14N) and O (18O/16O) in NO-3 can be used to aid in distinguishing various sources of NO-3 in groundwater effectively because values of δ15N-NO-3 and δ18O-NO-3 in groundwater vary with the sources of nitrate. Different physical, chemical and biological processes may lead to distinctive nitrogen and oxygen isotope fractionations. When denitrification occurs in groundwater, the enrichment for 15N in NO-3 is higher by a factor of about 2.0 than for 18O in NO-3. Thus, a dual isotopic approach of measuring both δ15N and δ18O in NO-3 could be employed to trace nitrate cycles in groundwater, especially the occurrence of denitrification. Use of both N and O isotopes in NO-3 combined with other environmental isotopic and chemical methods to trace nitrate sources and cycles are an important way in the investigation of nitrate contamination of groundwater. As to techniques for the measurement of 15N/14N and 18O/16O compositions of nitrate, a newly developed method for nitrate concentration using anion exchange resins has been applied in more and more studies because it is convenient and economical. In this paper, the progress of identifying NO-3 sources and its cycles in groundwater with the aid of δ15N-NO-3 and δ18O-NO-3 is reviewed. Future possible developments are put forward along with some problems in this research field.