The aim of this volume of essays is to delineate and examine a range of topics which represent a systematic account of the nature and implications of QFT.The contributors, who include Michael Redhead, James T. Cushing, Paul Teller, and Gordon Fleming, approach QFT from a variety of standpoints. Part I offers two different interpretations of the value of studying the foundations of QFT as an area of separate metaphysical research. Parts II and III consider the metaphysical and methodological implications of such issues as the problem of the status of virtual particles; the technique of renormalization; and the role of covariance principles. Part IV examines the mathematical foundations of QFT.