This volume contains the proceedings of a symposium held in St. Vincent, Italy, on August 3-6,1976, entitled "Vigilance II: Relationships Among Theory, Physiological Correlates, and Opera tional Performance." The symposium was sponsored jointly by the Human Factors Panel of the Scientific Affairs Division of NATO and by the Physiology Programs Office of the U.S. Office of Naval Research (Contract N00014-76-C-0722). These sponsorships, and the helpful assistance of Dr. Donald P. Woodward and Dr. John A. Nagay, are gratefully acknowledged. Following the editor's introduction, the papers appear in the order they were given at St. Vincent. In general they are grouped according to the main topical themes of the symposium: keynote ad dress, vehicle operation, monitoring and inspection, physiological correlates, stress effects, individual differences, and theoretical considerations. I must point out, however, that individual papers often overlapped several of these topical areas and thus no defini tive partitioning of the proceedings has been attempted. I wish to acknowledge the many contributions of my colleague, Dr. James O'Hanlon, to both the planning of the symposium and the preparation of the manuscript, and the invaluable role of my wife, Shirley Jean, in making the symposium such a pleasant one for all. In addition, I wish to recognize the countless contributions of Lynda Lee Chilton and Katherine Peimann whose performance in compos ing, typing, and proofing the manuscript is certainly a tribute to human vigilance, and endurance as well.