The Declaration of Helsinki was originally adopted by the World Medical Association (WMA) in 1964, and has undergone six revisions. The most recent revision was made in October 2008 in Seoul, Korea. The Declaration of Helsinki is the most widely known international guideline to provide the ethical principles for medical research involving human subjects. As the number of clinical researches has been increasing markedly in Korea, there are growing expectations and concerns about the research ethics related to the protection of human subjects. Physicians conducting the clinical research are supposed to know and understand the content of the Declaration, and they should follow the guideline in their researches. The current structure of the Declaration has three parts: Introduction; Basic Principles for All Medical Research; and Additional Principles for Medical Research Combined With Clinical Care. It includes essential elements of the ethical principles for clinical research, such as informed consent, research ethics committee, and risk-benefit assessment. Although there have been continuing controversial issues in the Declaration of Helsinki, it will remain to be the cornerstone of research ethics.