Although there are those who would deny it, the body of psychoanalytic knowledge and theory, particularly in regard to metapsychology and ego psychology, has undergone a profound and significant advance in the last quarter-century. Beginning perhaps with Freud's last major paper, "Analysis Terminable and Interminable" (1937), and Anna Freud's major work, "The Ego and the Mechanisms of Defense" (1936), a small but vital number of writers have explored the vicissitudes, structures, and functions of the concept of the ego. Not the least of the important papers dealing with this subject are those by Heinz Hartmann, Ernst Kris, and Rudolph M. Loewenstein (to be published soon as a separate monograph of Psychological Issues), and those in this present volume, a collection of significant essays by Heinz Hartmann. All of these have been previously published elsewhere, but the publisher, by collecting them under one cover, and providing them with a summary-introduction