All Korean citizens should join the National Health Security System by law. The National Health Insurance Service (NHIS) and the Health Insurance Review and Assessment Service (HIRA) are one of major components to support this system, and all data about medical expenses for the medical claims are stored and managed in the institutions. Recently, medical research using administrative claims databases has dramatically progressed in Korea and worldwide, and the methods how to use them are briefly reviewed in this article. Research using these databases have several strengths. Researchers can perform the complete enumeration survey in a real world. They can get new valuable findings because the number in the database is usually large enough to detect the minute difference with a big statistical power. They can obtain more detailed and reproducible results. Moreover, they can investigate a very rare disease or infrequent side effects of drugs. However, we must recognize that research using administrative claims database also has several incoherent limitations. These databases have not been constructed originally for research, but for reimbursement. Therefore, there are no important data including medical history and laboratory findings of each patient, which are crucial to adjust baseline characteristics. In addition, it is hard to discover causal relationship and direct association with the included information. In spite of limitations, researchers can easily use these databases for their research now than ever, and the results may be utilized not only to expand the academic knowledge but also to influence the determination of national healthcare policy.