The fourth Advanced Study Institute (ASI) on Techniques and Concepts of High Energy Physics was held once again at the Hotel on the Cay, in the scenic harbor of Christiansted, St. Croix, U.S. Virgin Islands. The ASI brought together a total of 67 participants, from 17 different countries. It was a great success, due to the dedication of the inspiring lecturers, the exceptional student body, and, of course, the beautiful setting. The primary support for the meeting was again provided by the Scientific Affairs Division of NATO. The ASI was cosponsored by the U.S. Department of Energy, by Fermilab, by the National Science Foundation, and by the University of Rochester. A special contri­ bution from the Oliver S. and Jennie R. Donaldson Charitable Trust provided an important degree of flexibility, as well as· support for worthy students from developing nations. As in the case of the previous ASI's, the'scientific program was designed for advanced graduate students and recent PhD recipients in experimental particle physics. The present volume of lectures should complement the material published in the first three ASI's, and prove to be of value to a wider audience of physicists. It is a pleasure to acknowledge the encouragement and support that I have continued to receive from colleagues and friends in organizing this meeting. I am indebted to the members of my Advisory Committee for their infinite patience and excellent advice. I am grateful to my distinguished lecturers for participating in the ASI.