The World Medical Association's Declaration of Helsinki was first adopted in 1964. In its 40-year lifetime the Declaration has been revised five times and has risen to a position of prominence as a guiding statement of ethical principles for doctors involved in medical research. The most recent revision, however, has resulted in considerable controversy, particularly in the area of the ethical requirements surrounding placebo-controlled trials and the question of responsibilities to research participants at the end of a study. This review considers the past versions of the Declaration of Helsinki and asks the question: How exactly has the text of the Declaration changed throughout its lifetime? Regarding the present form of the Declaration of Helsinki we ask: What are the major changes in the most recent revision and what are the controversies surrounding them? Finally, building on the detailed review of the past and present versions of the Declaration of Helsinki, we give consideration to some of the possible future trajectories for the Declaration in the light of its history and standing in the world of the ethics of medical research.