There are five major reasons why the physicist should know and be interested in the history of physics: (1) the great physicists have left us in their writings a treasure containing valuable suggestions for future laws, principles, theories, and experiments; (2) it is necessary to know why theories are now, or have been, discarded and are in disrepute: frequently a hostile scientific atmosphere may have been the sole reason why an important theory was rejected in the past; (3) in physics, many historical theories have been revived and applied anew; (4) the original historical arguments given in support of a theory may be invalid; and (5) physicists can often obtain inspiration by studying the lives of great men.