This introduction presents an overview of the key concepts covered in the subsequent chapters of this book. The book presents psychoanalytic thinking about the phenomenon of the couple and couple dynamics in internal and external reality and at different levels of organization: the "couple" in the individual's internal world, the dynamics between partners in a couple relationship, and the dynamics between the couple and the group. Bion states that "the group can be regarded as an interplay between individual needs, group mentality, and culture", and the incentive for the book was to explore thinking about the couple in different fields of observation and in the interplay between them. Theories derived from psychoanalytic work with individuals are applied in thinking about couple dynamics. States of mind in the individual can fluctuate between primitive and more mature ways of relating, and the dynamics in the couple can fluctuate accordingly.