Tenure in American medical schools is a personnel policy which exists in relation to institutional policies and realities. It is related to the functions of academic institutions and their capacity for carrying out these functions, and it arises from the needs of an open society to sustain critical and innovative roles for its universities, the necessity for universities to support and protect their faculties, the optimization of their capability to react to new opportunities, and the demands of ever shifting social circumstances. The relation between society and its universities and the balance among the various roles of the universities have not been stable in the past and will not be so in the future. Tenure policies will continue to be modified. The leadership of institutions needs to be aware of the fact that the modifications will occur and of what might be termed the plasticity of tenure.