One of the functions of any society is to protect its members from extreme forms of physical stress, such as cold, tissue damage, pain, and infectious disease. These are the types of stresses that have been studied most intensively in the laboratory, especially by the brilliant work of Selye (1956). However, as our modern technological societies have become better at protecting all but the most disadvantaged of their members from such physical stresses, other more psychological types of stress that involve learning become relatively more important. In the first part of this paper I shall concentrate on one of the best understood of these, namely, fear and the physiological responses to it. In the second part I shall deal with attempts to apply learning more directly to the modification of physiological responses.