This introduction presents an overview of key concepts discussed in the subsequent chapters of this book. The book reviews the "Strange Situation" studies conducted by Mary Ainsworth, research that substantiated and elaborated Bowlby's hypotheses about the impact of relational security on infant behavior. It traces the extension of attachment theory into the study of child and then adult development and functioning in close relationships. The book discusses the work of theorists and researchers in the relatively new field of interpersonal neuroscience, work that has strengthened the scientific foundation of attachment theory. It proposes the relationship between therapist and client is paramount in both types of therapy, an explicit focus on the patient's transference is a significantly less frequent occurrence in grief therapy than in other types of psychotherapy. It is worth noting that the chapter sees differences between therapy that focus on chronic attachment based problems in interpersonal relationships, and the practice of attachment-informed grief therapy.