I would hope that after reading this study the reader will have an increased appreciation of the varied and important roles that experiment plays in physics. The reader should conclude that experiment has a philosophically legitimate role in the choice between competing theories and in confirmation of theories or hypotheses and that there are good reasons for believing in experimental results. These are rather commonplace and obvious conclusions. Why should one write or, perhaps more important, read a book that concludes so little? I believe that detailed historical study of actual, as opposed to mythical, experiments is worthwhile. Some philosophers and sociologists of science have sought to deny, or to minimize, the role that experiment plays in theory choice and to cast doubt on the validity of experimental results. This book is my attempt to answer them.