Human dignity is the supreme criterion for protecting research participants, and likewise for numerous ethical matters of ultimate importance. But what is meant by “human dignity”? Isn’t this some vague criterion, some sort of lip service of questionable relevance and application? We shall see that it is nothing of the sort, that to the contrary, it is a very definite and very accessible criterion. However, how is this criterion applied in protecting research participants? These are the matters that we will examine now. My presentation is divided into four parts. 1/Recognizing Human Dignity; 2/Practical Definition of Human Dignity; 3/The Human Being in a Weakened State; 4/ Conclusion.